Ok, I'll bite for just one more time. Let's forget the academics and other human philosophies for a second (since they ultimately boil down to "he said... she said" debates in the end).
At 1 Corinthians 15, Paul speaks of the importance of the resurrection of the Christ, of the first Adam and "the last Adam", and of fleshly and spiritual bodies and how God chooses to clothe ones in one body or the other as it pleases him.
For anyone rejecting the view of Jesus as a completely separate being from the Father, how could Jesus become an equivalent of Adam if he is co-eternal and part of an immutable Godhead?
Adam was non-existent before being created from the dust. He had before him the risk of dying and ceasing to exist again (returning to dust) if he failed God and chose to sin (which he did).
If Jesus has always existed as a part of a Godhead, he could not possibly be a parallel to Adam. How could his test in the flesh work? If he had failed by choosing to sin and then died in the flesh, he would simply have returned to being that part of the Godhead (or possibly even continued to be so in one form while also still in the flesh - depending on one's interpretation of how this Godhead is supposed to be).
However, because he was, in fact, a separate being, created by the Father first in spirit form, then transformed by the Father into flesh, his failure could have had the same consequences as that of Adam's - complete destruction. It also makes his success more directly relatable to all humankind (1 Peter 2:21), proving that they too could choose to be obedient if raised back to the same perfect state as Adam (and Jesus in the flesh) had been.
The parallel and symbolic importance is clear:
The first Adam was a physical created son of God who proved unfaithful as a father to mankind and thus condemned his offspring to death, the "last Adam" (Jesus) a spiritual created son of God who then took on the same fleshly mantle but who proved it was possible to be faithful while in the flesh, and also thus became the adoptive father to all whom Adam had condemned to death (and hence became the promised "Eternal Father" - Isaiah 9:6 - often confused with God).
Indeed, at death Jesus ceased to exist, being "among the dead" until the "third day" (Luke 24:21,46; John 20:9), until his Father raised him up (John 10:17,18; Acts 2:24,32) back to the flesh (John 20:17), then later he returned to the spirit world (Luke 24:51, Acts 1:9). How could he be "among the dead" if he was a co-eternal part of a Godhead?
