According to those who deny Jesus is God, In Stephen's discourse in Acts 7:2. Who appeared to Abraham and what form did he take?

This is a good question, answerable from Scripture. Let's take a look.

And he said, Men, brethren, and fathers, hearken; The God of glory appeared unto our father Abraham, when he was in Mesopotamia, before he dwelt in Charran, (Acts 7:2, KJV)

The Greek Verb

The Greek word "ὤφθη" that has been translated here as "appeared" may not mean what it first appears (no pun intended).

Consider how this same word, Strong's number G3700, is used in other places.

As the above text table demonstrates, the Greek word for "see" can be used with broad application, just as its English equivalent is used, ranging from seeing with the eyes to seeing with the understanding. It can be applied to seeing something in a dream, in person, or in theory (e.g. "see the salvation of God").

Stephen, therefore, cannot be made to say, or mean, that Abraham had literally seen God. That his speech applies to having figuratively seen God is clear from the fact that no one, according to John, has ever seen God.

No man hath seen God at any time, the only begotten Son, which is in the bosom of the Father, he hath declared him. (John 1:18, KJV)

No man hath seen G2300 God at any time. If we love one another, God dwelleth in us, and his love is perfected in us. (1 John 4:12, KJV)

Note: The Greek word used in 1 John for "hath seen" is always used throughout the New Testament with application to the sight of the eyes; i.e. personal witness via the sense of sight. It is never used in a figurative or euphemistic sense, only in its literal sense. See more of this HERE.

However, the Greek word used in John 1:18, "ἑώρακεν" (G3708), is of similar root to the word used by Stephen, and can be used figuratively, such as in the sense of "take heed," or in the sense of understanding, though it usually applies to seeing with one's eyes.

Ye see G3708 then how that by works a man is justified, and not by faith only. (James 2:4, KJV)

Even in the sense of understanding, of course, it is not possible that any finite human has fully comprehended God, so John is correct to say that no one has ever "seen" God. But that John uses this same expression twice, with a different verb each time, and that John's usage in the latter case expressly applies to the sight, is instructive. If no one has ever seen God, Abraham is included.

Stephen's verb, G3700, is the broadest of these Greek terms in its potential application. It can address an encounter, a perception, or an experience. It is certainly not limited to a literal eye-witness sense of "seeing."

The Hebrew Verb

The Hebrew word used in Genesis 17:1-2 for "appeared" is the word "וַיֵּרָ֨א/way·yê·rā" (H7200). It can mean "to present oneself"; and it is used in Genesis 12:1 of God speaking to Abram to say "I will shew thee" (KJV). The same word is used in the prior chapter, and in a few chapters earlier, in a sense of finding out.

And the LORD came down to see H7200 the city and the tower, which the children of men builded. (Genesis 11:5, KJV)

And out of the ground the LORD God formed every beast of the field, and every fowl of the air; and brought them unto Adam to see H7200 what he would call them: and whatsoever Adam called every living creature, that was the name thereof. (Genesis 2:19, KJV)

Further expanding on the flexibility of this verb, again from the same book of the Bible, same writer, same Hebrew language, and even the same Abraham, we have the following text:

And Abraham said, My son, God will provide H7200 himself a lamb for a burnt offering: so they went both of them together. (Genesis 22:8, KJV)

This latter usage parallels that of Genesis 17:1-2 in that both verbs are of the imperfect form in Hebrew.

Conclusion

While the text makes explicit reference to Yahweh, the question is premised on Abraham having seen God; whereas the Biblical language does not limit it to this application. Abraham may well have encountered God in some manner, without entailing his having seen God's person. In fact, if John is to be believed, Abraham cannot have actually seen God, so God's "appearance" to Abraham must be understood in a figurative sense.