I think there was a pre-existent reality that was a possibility where he could have."
I find this to be nonsequiter jibberish!
"How to fully explain this, I have to leave to greater minds than mine."
You said id, so it is up to you, who created it, to explain your imaginary mechanism.
Actually I have the answer, but you have to pass Theology 101 to get it.
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.
I see both of you want to conveniently ignore it.