Jesus Christ, Son of God, the Word become flesh...
If you mean to say by "Lord" the same as God Incarnate; God, The Son, then I don't see the difference here. If not, aren't you introducing a fourth person?
Do you see my point?
If you want to say the Lord did not have a physical appearance prior to Mary, then you negate all the physical descriptions of the Lord throughout the Old Testament.
People obviously ate and drank with Christ prior to the Incarnation so that can't be right. However, no one that I know of run around with sapphires under their toes. Of course I might be running with the wrong crowd.
At the risk of minimizing the Incarnation, the only thing that happened was our Lord Jesus took on the form of us, identifying Himself physically with His creatures. Still God, but now man. How precisely He did this is a mystery but He chose to do it through a virgin; a virgin who needed a Savior just like everyone else for we all have sinned and fall short. Mary and Joseph didn't even have a chose in naming Him.
Now why you would like to believe Mary was sinless I don't know. Our Lord Jesus chose to be thrown into a fithy, vile, corrupt environment for 33 years; tempted in every way (probably more) than we are, yet without sin. Why Catholics think Mary had to be a pure environment so that our Lord Jesus could come into this world is the real mystery.
What do you mean by the "Mother of God"?