Which Vedic verses describe Indra as Brahman? Or as creator of universe or as source of universe or as someone who pervades whole universe?
Indra is called as the Lord of the Universe, the all pervader and the Supreme Lord in many Vedic verses. Here are some examples from RigVeda:
रूपं-रूपं परतिरूपो बभूव तदस्य रूपं परतिचक्षणाय |
इन्द्रो मायाभिः पुरुरूप ईयते युक्ता हयस्य हरयःशता दश || (RigVeda 6.47.18)
In every figure he hath been the mode: this is his only form for us to look on. Indra assumes many form by his Maya, for his Bay Steeds are yoked, ten times a hundred.
यो विश्वस्य जगतः पराणतस पतिर्यो बरह्मणे परथमो गा अविन्दत |
इन्द्रो यो दस्यून्रधरानवातिरन म... || (RigVeda 1.101.5)
He who is Lord of all the world that moves and breathes, who for the Brahman first before all found the Cows; Indra who cast the Dasyus down beneath his feet,—him girt by Maruts we invoke to be our Friend.
Let us praise him who made these worlds and creatures, all things that after him sprang into being. May we win Mitra with our songs, and Indra, and. wait upon our Lord with adoration. (RigVeda 8.85.6)
Carrying your power, bear your hymns of affirmation to Indra as the Truth, if in truth he exists. ” There is no Indra, some say to you. Who has seen him? Why should we praise him. ” I am here, O singer, perceive me here. I transcend all Beings by my greatness, the directions of truth increase me, as the one who breaks things open, I break open the worlds.” (Rig Veda 8.89.3-4)
Indra extends beyond heaven and earth. Half of him is equal to both the worlds. That is the truth Indra there is no other God or mortal greater than you. You are the king of the creatures of the world. You generated together the Sun, heaven and the Dawn. –(Rig Veda 6.30.1,4,5.)
Similarly, in the Taittariya Aranyaka of Krishna YajurVeda (in the part of MahaNarayaniya) it is stated:
VIII-1: The Supreme Being, Indra, who is the most excellent Pranava taught in the Vedas, who ensouls the entire universe, who leads the collection of Vedic utterances in Gayatri and other metres standing in their beginning, who is capable of being attained by the worshippers and who is the first in the causal link, taught the contemplative sages the sacred wisdom of the Upanishad, Himself being the subject-matter of them, in order to strengthen them with the power of knowledge. I salute the gods for removing the obstacles in my path to illumination. For the same I also reverence the Manes. The triple regions of Bhuh, Bhuvah and Suvah and the entire Veda are comprised in Om.
Similarly in the Kaushitki Aranyaka of RigVeda (where there is Kaushitki Brahmana Upanishad) it is told:
To him then Indra said: A superior verily chooses not for an inferior. Do you yourself choose. ‘No boon verily then is it to me’ said Pratardana. But Indra did not depart from the truth, for Indra is truth. To him then Indra said: ‘Understand me only. This indeed I deem most beneficent to man, namely that one should understand me. I slew the three-headed Tvastir; I delivered the Arunmukhas, the ascetics, to the wolves. Transgressing many compacts I killed the people of Prahlada in the sky, the Paulomas in the atmosphere, the Kalakanjas on the earth. Of me, such as I was then, not a single hair was injured. So he knows me thus – by no deed whatever of his is his world injured, not by stealing, not by killing an embryo, not by the murder of his mother, not by the murder of his father. If he has done any evil, the dark colour departs not from his face.
III-2. Then he (Indra) said: I am the Spirit of the vital breath, the intelligent Self. As such, worship me as life, as immortality. Life is the vital breath: the vital breath is life. For as long as the vital breath remains in the body so long is there life. For indeed with the vital breath one obtains immortality in this world; with intelligence, true conception. So he who worships me as life, as immortality, reaches the full term of life in this world; he obtains immortality and indestructibility in the heavenly world.
As a sidenote regarding the interpretation of such verses in the Vedanta, it is discussed in Brahma Sutras as;
28. Prana is Brahman, because it is comprehended thus.
29. If it be argued that Prana is not Brahman, since the instruction is about the speaker’s own self, (then we say, no), for here is an abundance of reference to the inmost Self.
30. But the instruction proceeds from a seer’s vision agreeing with scriptures, as in the case of Vamadeva.
31. If it be argued that Brahman is not spoken of here on account of the indications of the individual soul and the chief vital force, then that cannot be so, since this will lead to a threefold meditation. (Besides, Prana) is accepted (elsewhere) as meaning Brahman (because of the presence of Brahman’s characteristics), (and these are) in evidence here.
The passage being discussed in the Brahma Sutra above is the same passage of Kaushitki Brahmana Upanishad which says Indra is Brahman. Here is commentary of Adi Shankaracharya on that part:
The individual divine Self called Indra perceiving by means of rishi-like intuition 1--the existence of which is vouched for by Scripture--its own Self to be identical with the supreme Self, instructs Pratardana (about the highest Self) by means of the words 'Know me only.'
By intuition of the same kind the rishi Vâmadeva reached the knowledge expressed in the words, 'I was Manu and Sûrya;' in accordance with the passage, 'Whatever deva was awakened (so as to know Brahman) he indeed became that' (Bri. Up. I, 4, 10). The assertion made above (in the pûrvapaksha of the preceding Sûtra) that Indra after saying, 'Know me only,' glorifies himself by enumerating the slaying of Tvashtri's son and other deeds of strength, we refute as follows. The death of Tvashtri's son and similar deeds are referred to, not to the end of glorifying Indra as the object of knowledge--in which case the sense of the passage would be, 'Because I accomplished such and such deeds, therefore know me'--but to the end of glorifying the cognition of the highest Self. For this reason the text, after having referred to the slaying of Tvashtri's son and the like, goes on in the clause next following to exalt knowledge, 'And not one hair of me is harmed there. He who knows me thus by no deed of his is his life harmed.'--(But how does this passage convey praise of knowledge?)--Because, we reply, its meaning is as follows: 'Although I do such cruel deeds, yet not even a hair of mine is harmed because I am one with Brahman; therefore the life of any other person also who knows me thus is not harmed by any deed of his.' And the object of the knowledge (praised by Indra) is nothing else but Brahman which is set forth in a subsequent passage, 'I am prâna, the intelligent Self.' Therefore the entire chapter refers to Brahman.
Ramanujacharya commentary on the same:
As the Rishi Vâmadeva perceiving that Brahman is the inner Self of all, that all things constitute its body, and that the meaning of words denoting a body extends up to the principle embodied, denotes with the word 'I' the highest Brahman to which he himself stands in the relation of a body, and then predicates of this 'I' Manu Sûrya and other beings--'Seeing this the Rishi. Vâmadeva understood, I am Manu, I am Sûrya' (Bri. Up. I, 4, 10). Similarly Prahlâda says, 'As the Infinite one abides within all, he constitutes my "I" also; all is from me, I am all, within me is all.' (Vi. Pu. I, 19, 85.) The next Sûtra states, in reply to an objection, the reason why, in the section under discussion, terms denoting the individual soul, and others denoting non-sentient things are applied to Brahman.
And Srikantha Shivacharya in his commentary states:
The Sutrakara quotes an example, "like Vamadeva." Vamadeva saw that Paramesvara was none but his own Atma and exclaimed " I have become Manu and Surya." Just so is Indra's declaration.
Or thus : When, by the contemplation of the harmonious nature of Brahman and Atman brought about by Vedantic knowledge, Vamadeva attained to the state of Brahman and was freed from all the imaginary limitations due to the identifying of himself with the human body and so on, and his mighty ego expanded so as to embrace the whole universe, he saw that he was present everywhere and accordingly spoke of himself as one with the whole univierse including Manu and Surya. So, it maybe concluded, it was in the case of Indra. In the passage " I am prana, the conscious Atman," Prana refers to Para-Brahman, in as much as He , blissful by nature, is the cause of all life, as said in the sruti " Prana is the conscious self, the Bliss, undecaying and immortal." Accordingly it is from the standpoint of Brahman that Indra taught " I am Brahman," " me do thou worship " So, too, Krishna taught to Arjuna, and so several others.
Brahman means knowing all. Knowledge. Lord Indra is the param Brahman that knows everything. As I understand Vishnu Brahma and Shiva are force of existence, they are the parameters and platforms. Consider, they are The inherent program of a PC that you can’t see, or experience directly. They are the software code. Indra is like Windows. Every other program runs in windows, without which you can’t do anything. Rest of the Gods are like MS office, Photoshop, Coreldraw etc that are empowering in their right. But without Windows they can’t do anything.
Krishna is supposedly how the essence of universe it’s collectve consciousness takes up a form. I’m not sure why, maybe to elevate or guide humanity. Because Gods won’t guide humans to their greatness or their cycles. That interest lies with Krishna.