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It is well known that Vaishnavas and the scriptures likewise identify Lord Vishnu as the Supreme or Brahman (*1). So someone may try to interpret those statements which I quoted above saying "he becomes Vishnu" as if they say that jivatma will become Lord Vishnu, or alternatively if word "vishnu" is taken to mean "All-pervader" as denoting not God Vishnu but Brahman who is said to be omnipresent in the scriptures, then Vaishnavas would say that both of these interpretations are not possible.
It is not possible for a jivatma to become Lord Vishnu simply because He is one god (*1) who eternally (*2) holds that position, and therefore no one can replace him and take over his position.
And that other interpretation according to which a jivatma is supposed to become "all-pervader" also makes no sense because the jivatma is said to be atomic in size (*3), and also is said to be unchangeable.
It is stated in the Bhagavad gita 2.25 that the soul is avyakto ’yam acintyo ’yam avikāryo ’yam ucyate, where the word avikāryo mean "unchangeable", it means that the soul cannot change its size because if it changes its size it would not be avikārya. So it makes no sense to think that the jivatma can change its size from being small atomic particle to became huge soul "all-pervading" everything, ie to became literally omnipresent in its size.
Also it's worth to mention that the scriptures say the jivatma is eternal (*4), so he cannot cease to exist (*5). The jivatma is eternally small atomic particle of spirit, and it stays to be like that always, never changing its size. Even when he gets moksha he will stay to be such forever.

It is well known that Vaishnavas and the scriptures likewise identify Lord Vishnu as the Supreme or Brahman (*1). So someone may try to interpret those statements which I quoted above saying "he becomes Vishnu" as if they say that jivatma will become Lord Vishnu, or alternatively if word "vishnu" is taken to mean "All-pervader" as denoting not God Vishnu but Brahman who is said to be omnipresent in the scriptures, then Vaishnavas would say that both of these interpretations are not possible.
It is not possible for a jivatma to become Lord Vishnu simply because He is one god (*1) who eternally (*2) holds that position, and therefore no one can replace him and take over his position.
And that other interpretation according to which a jivatma is supposed to become "all-pervader" also makes no sense because the jivatma is said to be atomic in size (*3), and also is said to be unchangeable.
It is stated in the Bhagavad gita 2.25 that the soul is avyakto ’yam acintyo ’yam avikāryo ’yam ucyate, where the word avikāryo mean "unchangeable", it means that the soul cannot change its size because if it changes its size it would not be avikārya. So it makes no sense to think that the jivatma can change its size from being small atomic particle to become huge soul "all-pervading" everything, ie to become literally omnipresent in its size.
Also it's worth to mention that the scriptures say the jivatma is eternal (*4), so he cannot cease to exist (*5). The jivatma is eternally small atomic particle of spirit, and it stays to be like that always, never changing its size. Even when he gets moksha he will stay to be such forever.

Verily in the beginning this was Brahman, that Brahman knew (its) Self only, saying, 'I am Brahman.' From it all this sprang. Thus, whatever Deva was awakened (so as to know Brahman), he indeed became that (Brahman); and the same with Rishis and men. The Rishi Vâmadeva saw and understood it, singing, 'I was Manu (moon), I was the sun.' Therefore now also he who thus knows that he is Brahman (another translator: "I am Brahman"), becomes all this, and even the Devas cannot prevent it, for he himself is their Self (another translator: for he becomes their self).

The above Brihadaranyaka Upanishad 1.4.10 and Mundaka Upanishad 3.2.9 verses also seem not to support Advaita of Shankara just like the above Mahabharata text. And this is so because per the Shankara's Advaita we already are Brahman and thus it makes no sense to say that we will "become" Brahman. Actually when I think about those verses it sounds to me as if they are written according to some Vaishnava system of Vedanta. This is so because it is possible to "become Brahman" in Vaishnava systems of Vedanta and stay to be an individual (jivatma) even when you become Brahman. Of course, to "become Brahman" in Vaishnava understanding doesn't mean to lose individuality and to become literally the Supreme. The expression "become Brahman" Vaishnavas interpret differently than Advaitins.
Vaishnavas think that scriptural statements of that kind, saying something like "he will become Brahman" or "he will become the inner self of all (paramatma)", simply means that the jivatma will enter into Brahman like a fish is thrown into the ocean, and thus the fish will "become one with" the ocean, ie he will stay to be a small atomic part of an huge ocean. So in this sense a fish is the ocean when he enters into it. It's not that a fish is a whole ocean. And also the fish doesn't lose his individuality (jivahood, to be a jivatma) in the ocean. In this sense the statements "he will become Brahman/paramatma" or even "he becomes Vishnu" and "I am Brahman" should be understood. It's like a fish would say "I will become an ocean" when thrown into it, and when he finally enters an ocean the fish can say "I am the ocean", which is equivalent to the statement "I am Brahman".
There is a nice example of this in the Mundaka Upanishad 2.2.4:

This is similar to the above example I said about the fish which is thrown into the ocean. Here the jivatma is like an arrow which hits the target (Brahman), and thus literally enters the target (Brahman), and stays like that in the target, ie in Brahman. Thus even when an arrow enters the target, it stays in it and the difference between the arrow and target still exist although we could say that they have become one, we could say "an arrow had become the target". Similarly we say for the fish or the jivatma, "he becomes Brahman". Thus the individuality of the jivatma will continue even when the jivatma enters into the pure existence of Brahman. Just like an arrow will continue to exist after it hits the target, so the jivatma likewise. To attain Brahman doesn't mean to lose individuality.

So the conclusion:
It's more likely those statements support Vaishnava systems of Vedanta, because in them you can "become Brahman" like a fish can become one with the ocean and still you retain your individuality, as I explained.

-- (*1) Narayana Upanishad says: "Nārāyaṇa who pervades all elements, who is one only, who is the cause Purusha and who is causeless, is known as Parabrahman." So, the verse says there is only one Lord Narayana, namely there are not two, three, or more gods Narayana. He is called Brahman or Parabrahman (Supreme Brahman) in this verse also. For this reason it makes no sense to think that jivatma can become yet another Lord or God Narayana. Thus statements that I quoted above such as "Whoever knows Him thus, becomes Vishṇu Himself." cannot be interpreted to mean that jivatma will become Lord Vishnu (Narayana), but should be interpreted in the sense as I explained, namely it means that the jivatma can become a resident (inhabitant) living in Vishnu or Brahman when he gets moksha and enters into Brahman, like a fish which enters an ocean can become a resident (inhabitant) of an ocean, and thus living in the ocean a fish can say "I have become an ocean".

d) Katha Upanishad chapter 6, verse 16 says: "There are a hundred and one arteries of the heart, one of them penetrates the crown of the head. Moving upwards by it, a man (at his death) reaches the Immortal; the other arteries serve for departing in different directions."
In this verse it is explained how the jivatma is going out of the body at death. The jivatma is located in the heart of the body. And when a man dies the jivatma have to start moving from the heart through one of those arteries (nadis), as the verse says "There are a hundred and one arteries of the heart", and the verse says that the jivatma can depart from the body passing through an artery that ends at the crown of the head. So the jivatma must go through an artery from the heart to the head. It indicates that the jivatma is small in size because if it's not small it would be impossible to go through an artery and travel like that. By the way, if we are not the small jivatma but the all-pervading omnipresent soul then there would be no need to travel through some artery and reach the next body for rebirth. The omnipresent soul could just somehow appear in the next body, there would be no need to travel to reach that body. A soul which is so much huge that is omnipresent, is located everywhere in the universe pervading this earth, heaven, whole universe and even beyond this universe. For such a soul there is no purpose to travel anywhere because that soul already is located everywhere. However, we see that the scriptures say every time when a person dies, he has to travel far distances to reach the next body for rebirth.

a) Bhagavad gita 15.7: mamaivāṁśo jīva-loke jīva-bhūtaḥ sanātanaḥ "The living entities in this conditioned world are My eternal fragmental parts."

Verily in the beginning this was Brahman, that Brahman knew (its) Self only, saying, 'I am Brahman.' From it all this sprang. Thus, whatever Deva was awakened (so as to know Brahman), he indeed became that (Brahman); and the same with Rishis and men. The Rishi Vâmadeva saw and understood it, singing, 'I was Manu (moon), I was the sun.' Therefore now also he who thus knows that he is Brahman (another translator: "I am Brahman"), becomes all this, and even the Devas cannot prevent it, for he himself is their Self (another translator: for he becomes their self).

The above Brihadaranyaka Upanishad 1.4.10 and Mundaka Upanishad 3.2.9 verses also seem not to support Advaita of Shankara just like the above Mahabharata text. And this is so because per the Shankara's Advaita we already are Brahman and thus it makes no sense to say that we will "become" Brahman as these verses say. Actually when I think about these verses it sounds to me as if they are written according to some Vaishnava system of Vedanta. This is so because it is possible to "become Brahman" in Vaishnava systems of Vedanta and stay to be an individual (jivatma) even when you become Brahman/paramatma/Lord Vishnu. Of course, to "become Brahman" in Vaishnava understanding doesn't mean to lose individuality and to become literally the Supreme. The expression "become Brahman" Vaishnavas interpret in a specific manner.
Vaishnavas think that scriptural statements of that kind, saying something like "he will become Brahman" or "he will become the inner self of all (paramatma)", simply means that the jivatma will enter into Brahman like a fish is thrown into the ocean, and thus the fish will "become one with" the ocean, ie he will stay to be a small atomic part of an huge ocean. So in this sense a fish is the ocean when he enters into it. It's not that a fish is a whole ocean. And also the fish doesn't lose his individuality (jivahood, to be a jivatma) in the ocean. In this sense the statements "he will become Brahman/paramatma" or even "he becomes Vishnu" and "I am Brahman" should be understood. It's like a fish would say "I will become an ocean" when thrown into it, and when he finally enters an ocean the fish can say "I am the ocean", which is equivalent to the statement "I am Brahman".
There is a nice example of this in the Mundaka Upanishad 2.2.4:

This is similar to the above example I said about the fish which is thrown into the ocean. Here the jivatma is like an arrow which hits the target (Brahman), and thus literally enters the target (Brahman), and stays like that in the target, ie in Brahman. Thus even when an arrow enters the target, it stays in it and the difference between the arrow and target still exist although we could say that they have become one, we could say "an arrow had become the target". Similarly we say for the fish or the jivatma, "he becomes an ocean" or "he becomes Brahman".(*6)
Thus the individuality of the jivatma will continue to exist even when the jivatma enters into the pure existence of Brahman. Just like an arrow will continue to exist after it hits the target, so the jivatma likewise. To attain Brahman doesn't mean to lose individuality.

So the conclusion:
It's more likely those scriptural statements support Vaishnava systems of Vedanta, because in them you can "become Brahman" like a fish can become one with the ocean, like an arrow can become one with the target, and still you retain your individuality, as I explained.

-- (*1) Narayana Upanishad says: "Nārāyaṇa who pervades all elements, who is one only, who is the cause Purusha and who is causeless, is known as Parabrahman."
So, the verse says there is only one Lord Narayana, namely there are not two, three, or more gods Narayana. He is called Brahman or Parabrahman (Supreme Brahman) in this verse also. For this reason it makes no sense to think that jivatma can become yet another Lord or God Narayana. Thus statements that I quoted above such as "Whoever knows Him thus, becomes Vishṇu Himself." cannot be interpreted to mean that jivatma will become Lord Vishnu (Narayana), but should be interpreted in the sense as I explained, namely it means that the jivatma can become a resident (inhabitant) living in Vishnu or Brahman when he gets moksha and enters into Brahman, like a fish which enters an ocean can become a resident (inhabitant) of an ocean, and thus living in the ocean a fish can say "I have become an ocean".

d) Katha Upanishad chapter 6, verse 16 says: "There are a hundred and one arteries of the heart, one of them penetrates the crown of the head. Moving upwards by it, a man (at his death) reaches the Immortal; the other arteries serve for departing in different directions."
In this verse it is explained how the jivatma is going out of the body at death. The jivatma is located in the heart of the body. And when a man dies the jivatma have to start moving from the heart through one of those arteries (nadis), as the verse says "There are a hundred and one arteries of the heart", and the verse says that the jivatma can depart from the body passing through an artery that ends at the crown of the head. So the jivatma must go through an artery from the heart to the head. It indicates that the jivatma is small in size because if it's not small it would be impossible to go through an artery and travel like that. By the way, if we are not the small jivatma but the all-pervading omnipresent soul then there would be no need to travel through some artery and reach the next body for rebirth. The omnipresent soul could just somehow appear in the next body, there would be no need to travel to reach that body. A soul which is so much huge that is omnipresent, is located everywhere in the universe pervading this earth, heaven, whole universe and even beyond this universe. For such a soul there is no purpose to travel anywhere because that soul already is located everywhere. However, we see that the scriptures say every time when a person dies, he has to travel first through an artery (nadi) to leave the body, and then far distances to reach the next body for rebirth. That strongly suggests that jivatma soul is small in size, namely atomic in size, as we also see from other scriptural examples which I'm quoting here.

a) Bhagavad gita 15.7: mamaivāṁśo jīva-loke jīva-bhūtaḥ sanātanaḥ "The living entities in this conditioned world are My eternal fragmental parts."
In this verse Lord Krishna clearly says that living entities (jivas) are eternal (sanātanaḥ). So individuality of the jivas is never destroyed, got lost or cease to exist.

-- (*6) Brihadaranyaka Upanishad 4.3.32 says: salila eko draṣṭādvaito bhavati | eṣa brahmalokaḥ samrāṭ | iti hainam anuśaśāsa yājñavalkyaḥ | eṣāsya paramā gatiḥ | eṣāsya paramā saṃpat | eṣo 'sya paramo lokaḥ | eṣo 'sya parama ānandaḥ | etasyaivānandasyānyāni bhūtāni mātrām upajīvanti
"An ocean becomes that seer (jivatma), without any duality; this is the Brahma-world, O King. Thus did Yâjñavalkya teach him. This is his highest goal, this is his highest success (or wealth), this is his highest world, this is his highest bliss (ānanda). All other creatures live on a small portion of that bliss."
This verse illustrates the point that a self-realised sage or jivatma becomes one with Brahman just like, say, a fish becomes one with the ocean, or just like an arrow becomes one with the target. That type of unity is especially seen in the verse to be expressed with the words "without any duality", which simply means that there is no more duality or separate existence between a released jivatma and Brahman, just like there is no more duality or separate existence between a fish who lives in the ocean and the ocean itself, or just like there is no more duality or separate existence between an arrow which is in the target and the target itself because they have become one, that is, they are in unity without duality.

d) Brihadaranyaka Upanishad 4.3.30 says "For knowing is inseparable from the knower, because it cannot perish."
In this verse an individual soul (jivatma) is called "the knower" because he has ability to know something, to be aware of something. From this statement we learn that the jivatma stays to be an individual for whole eternity, and also that jivatma's ability to know something, to learn or to be aware of something, also exists eternally. That eternality is expressed with the words "it cannot perish", ie it cannot die, vanish, fade away.

d) Brihadaranyaka Upanishad 4.3.30 says "For knowing is inseparable from the knower, because it cannot perish."
In this verse an individual soul (jivatma) is called "the knower" because he has ability to know something, to be aware of something. From this statement we learn that the jivatma stays to be an individual for whole eternity, and also that jivatma's ability to know something (called as "knowing" in the verse), to learn or to be aware of something, also exists eternally. That eternality is expressed with the words "it cannot perish", ie it cannot die, vanish, fade away. So neither the knower (an individual jivatma) will perish, nor his knowing (his ability to know something) will perish. Or that is to say both of them, the knower and his knowing, will never cease to exist.