Adam could not have fulfilled Christ's role, because Adam was not the incarnate God Himself. Only the Second Person of the Trinity can be God Incarnated--by the definition of the Trinity.
With regard to pre-existence, there is a kind of pre-existence of the elect. But this "pre-existence" (note the quotation marks!) is a spiritual idea (of being chosen in Christ before the foundation of the world) tied to:
1) the humanly illusory nature of time versus eternity; and
2) the more specific fact that our God, Who dwells in eternity, is a God of purpose.
I think our understanding needs to come right up the edge of these things. However, I maintain that when we get to the edge of these things, we must stop.
I hope this helps. Anyway, it's the way I have always looked at the matter.
Yet Adam was not forbidden to eat of the Tree of Life that stood in the middle of the garden. He was the firstborn among men, a right that gave him the title of priest, maybe even high-priest.We see from the book of Hebrews that Jesus fulfilled this role for us. And when Adam sinned, he had to be physically removed from the presence. He made his choice. Be like this tree or be like that tree. He wanted to be like that tree.