The transfiguration does not prove preexistence, it can however prove preeminence. Also your interpretation of John 17:5 contradicts Jesus words in John 17:3 as well as Isaiah where Yehovah the Father says:
Isaiah 42:8
8 "I am the LORD, that is My name;
I will not give My glory to another,
Nor My praise to graven images. NAS
Isaiah 48:11
11 "For My own sake, for My own sake, I will act;
For how can My name be profaned?
And My glory I will not give to another. NAS
Believer wrote:The Bible tells us in prophecy that the Lord Himself will teach us our ways. There are various other little verses that offer an interesting scene of God being with us. That's what Emmanuel means.
And how does God teach us His ways? The answer is through His prophets. Do you understand this prophecy?
Deuteronomy 18:15-19
15 The LORD your God will raise up for you a prophet like me from among you, from your countrymen, you shall listen to him. 16 This is according to all that you asked of the LORD your God in Horeb on the day of the assembly, saying, 'Let me not hear again the voice of the LORD my God, let me not see this great fire anymore, lest I die.' 17 "And the LORD said to me, 'They have spoken well. 18'I will raise up a prophet from among their countrymen like you, and I will put My words in his mouth, and he shall speak to them all that I command him. 19'And it shall come about that whoever will not listen to My words which he shall speak in My name, I Myself will require it of him. NAS
Jesus is Immanuel but that is a title not a description of the Lord's nature.
Jesus fulfilled this prophecy as He said:
John 7:16-17
16 Jesus therefore answered them, and said, "My teaching is not Mine, but His who sent Me. 17 "If any man is willing to do His will, he shall know of the teaching, whether it is of God, or whether I speak from Myself. NAS
John 17:7-8
7 "Now they have come to know that everything Thou hast given Me is from Thee; 8 for the words which Thou gavest Me I have given to them; and they received them, and truly understood that I came forth from Thee, and they believed that Thou didst send Me. NAS
You have not established the preexistence of the Son as part of a triune God. All you are doing is defending a tradition of men.