So God could make Mary sinless but needed a sinless woman to give birth because He couldn’t make Jesus sinless?
No. He gave Mary, who needed salvation, the singular grace of being preserved from original sin. Why? So she would be a pure vessel for the pure God-made-Man.
Jesus, by virtue of His divine nature was sinless. He didn’t need Mary or her consent. However, God chose to have His only Son come into the world, as man, born of woman.
As we read about the Annunciation in Luke 1:26-38, we see from the beginning, this is an unusual birth. God is to become man, born of woman. She is a virgin. We are also reminded that nothing is impossible with God.