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Wow....this is an excellent discussion. Thanks to the OP!
Remember that Christ was named 'Jesus' by the angel before he was conceived in the womb (Luke 2:21), so Jesus Christ 'is' pre-existant to this incarnation. So....
Jesus is a name given given to the pre-existant Christ born in Bethlehem. Jesus is a name and Christ is a title. Now the hard part.
The Bible says Jesus Christ is the same yesterday, today, and forever. (Heb 13:8) Hence, getting back to our OP's question....was he the God-Man (Deity/Human) before he was born? Well, it looks like he wasn't....
"....Christ Jesus: 6 Who, being in the form of God, thought it not robbery to be equal with God: 7 But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: 8 And being found in fashion as a man, he humbled himself.... (Phil 2:5-8)
The Bible says that "God is not a man, that he should lie; neither the son of man, that he should repent:...." (Numbers 23:19) Well, what we do know is that God exists eternally and has no beginning or end. So it would appear that the Son of God is eternal as the 2nd person of the Trinity, but the Son of man existed from his Incarnation on in to eternity future.
When Christ was 'made in the likeness of men'....he took on our human nature, he was "made to be sin for us" (2 Cor 5:21) so he could die on the Cross and set an example for us as the perfect man. He was not 'inherently of human nature but took it on (for us). It appears that Human Nature was 'added to' his Divine nature at the Incarnation. And after he died he was raised in a new body just as we will be.
Anyhow....it looks to me like before his Incarnation, he existed as only in the Divine nature as Deity (God). But I'm sure up for more discussion of this, in other words, other ideas of how he could have existed 'in the flesh' with human nature before his incarnation.
Also think of what's been mentioned already, that Rev 13:8 does say that he was the Lamb slain before the foundation of the world. And...we are made in the image of God (body, soul, spirit (three in One). Then again....the Bible tells us that God is a Spirit (John 4:24). Could the Lamb that was slain consist of the past 'thoughts' of God, with the 'ways' of God being the future reality?
So as you can see, I'm sure not clear on all this, so somebody else jump in and give us some more thoughts.