>> So a divine baby is more subject to contamination by sin through osmosis than a human baby? <<
ONCE AGAIN: THE ISSUE IS NOT CONTAMINATION. There is no sin in traipsing mud through a dorm room; there is sin in traipsing mud through the Temple. Likewise, Mary’s parents couldn’t sacrilege HER, because she was only a (very blessed, special) human. No need for her parents to be immaculate.
SIN is SIN. It is not what goes into a man that makes him sinful. It is what comes out of him - out of his mind, heart, and mouth.
Sin is lawlessness. Breaking the Law. What has that to do with Mary? Christ was sinless because of what came out of Him. He was full of the Law.
That we cannot approach the Father but through a mediator is for our sakes, not for YHWH's. We would perish in His full glory because of our sins. It is not a matter of His being defiled. He cannot be defiled.
Whatever condition that Christ is in comparison to the full glory of the Father, He is not that, lest every single thing that looked upon him would have perished. That obviously didn't happen.
So trying to say that Christ needed to be born of a sinless mother is simply absurd.