The bridge works both ways.
Christ is fully God and fully Man. When you are drowning, you need someone with their feet firmly on the other bank. You need God.
Christ before the incarnation was fully God, but he needed to become Fully Man, the same as Adam. Hence the miracle of the incarnation, where he became exactly this, the new Adam.
This is why it couldn’t be Mary, because Mary is not God.
“Why couldn’t God preserve himself from sin by being born of Mary.”
He could have taken on the likeness of Man, but it wouldn’t be the same thing. He had to take the flesh and be reformed of the flesh while he is in his mother’s womb. This means he no longer has control over the process once it’s started.
” This means he no longer has control over the process once it’s started.”
Sad. It’s very sad that you don’t believe in an all-powerful, sovereign, omnipotent God. Ben, God was, is, and never will be “out of control” over anything. He can do anything. Including being born of a virgin—a frail, human, sinful person just as in need of salvation as you or I.
Hoss
With errors in who Mary is, you’ll often find errors in who Christ is. The two tend to go together - which is why the Council that decreed Mary as Theotokos was all about Christology. Recall that, even using the same scripture, the Trinity was argued for centuries.
It continues today. Those who have a problem with Mary often exhibit heresy in their view of the Trinity: Nestorianism, etc.
If you don’t understand Mary, you don’t understand the Incarnation, and without that theology concerning Christ and the Holy Trinity is very often flawed.