First I'll focus on the claims regarding so-called sectarian Sruti.
For old Vedantists there were no such a thing called "sectarian Sruti". And those wiki statements about some Upanishads being Vaishnava, Shaiva, Shakta, old Vedantists would not accept as correct because for them all genuine Upanishads are non-sectarian, namely Upanishads are not connected with some particular sect.
The proof for that is well known Svetasvatara Upanishad which according to some people bears a designation of Shaiva Upanishad, but even Vaishnava Vedantists often have quoted from it in their writings. Had they believed Svetasvatara being a sectarian text dedicated to the Shaiva sect only, those Vaishnavas wouldn't ever quoted from it. But they did quote from that Upanishad, obviously because they thought Svetasvatara, and all the other genuine Upanishads they quoted from, are meant for Vaishnavas as well.
And that claim about the Maha Upanishad supposedly being a Vaishnava Upanishad, and thus acceptable only to the Vaishnavas, old Vedantists would not accept as correct because for them that Upanishad is Sruti expressing eternal Vedic truth and thus should be accepted by all.
There are verses in the Subala Upanishad which say Lord Vishnu, ie Narayana, is antaryami or the inner controller of all the other divinities, ie devas or gods. Thus Lord Narayana is the Supreme Self, Supersoul, or paramatma in the hearts of all the living beings (jivatmas), including all the divinities or gods in heaven who are rulers of certain aspects of this material universe. Being situated in the hearts of the divinities, Lord Narayana rules even over them.
Thus says Subala Upanishad:
Within the body, is the one eternal Aja (unborn), located in the cave (of the heart). Earth is His body. Though He moves in the earth, earth does not know Him. Waters are His body. Though He moves in the waters, waters do not know Him. Ṭejas is His body. Though He moves in ṭejas, ṭejas does not know Him. Vāyu is His body. Though He moves in vāyu, vāyu does not know Him. Ākāś is His body. Though He moves in ākāś, ākāś does not know Him. Manas is His body. Though He moves in manas, manas does not know Him. Buḍḍhi is His body. Though He moves in buḍḍhi, buḍḍhi does not know Him. Ahaṅkāra is His body. Though He moves in ahaṅkāra, ahaṅkāra does not know Him. Chiṭṭa is His body. Though He moves in chiṭṭa, chiṭṭa does not know Him. Avyakṭa is His body. Though He moves in avyakṭa, avyakṭa does not know Him. Akshara is His body. Though He moves in akshara, akshara does not know Him. Mṛṭyu (death) is His body. Though He moves in Mṛṭyu, Mṛṭyu does not know Him. Such an one is the Mind within all creatures, the remover of all sins and the Divine Deva, the one Nārāyaṇa.
In the above verse Subala Upanishad says that the one eternal Aja (unborn) soul is situated in the divinites of earth, waters, ..., etc, Mṛṭyu (death), and although that one eternal Aja (unborn) soul is situated in all those divinities, they do not know about him, ie they are not aware that he is in them. That the verse says by saying "Though He moves in the earth, earth does not know Him. ..., etc, Mṛṭyu does not know Him." And at the end the verse says, that one eternal Aja (unborn) soul is within all creatures, is the remover of all sins, is one Lord Narayana.
Subala Upanishad was quoted in the writings of old Vedantists such as Ramanuja, and even prior to him by Suresvara who was Shankara's disciple. As I already said the Upanishads which were quoted by the old Vedantists they regarded to be genuine Sruti expressing eternal Vedic wisdom, and not some sectarian texts.
There are similar verses to those which I quoted above, and they are located in the Antaryami Brahmana section of the Brihadaranyaka Upanishad, verses 3.7.3–23. In those verses it is also said that there is one soul, paramatma, in various divinities ruling over them all. For this reason that section is called Antaryami Brahmana, namely Antaryami means "the one who controls from within", ie it is paramatma who is within the divinities and thus he governs and controls them, or that is to say he is paramatma in their jivatmas ruling them from within them.
There is also a similar verse in another Sruti text called the Narayana Sukta in the Yajur Veda which says: nārāyaṇaṁ mahājñeyaṁ viśvātmānaṁ parāyaṇam. In that verse Lord Narayana is called as viśvātmānaṁ which means "the inner self (paramatma) of all beings", namely of all the beings including the divinities in heaven. The Narayana Sukta was also quoted by the old Vedantists including Shankara, Ramanuja, and others.