OK, I'll be logical.
According to you, Jesus was NOT God until after Mary had Him.
Got that right?
That is the ONLY way she could have been "Mother of God."
That term, by the way, DID exist long before Jesus was born. It referred to a pagan deity. As did "Queen of Heaven."
The RC church simply picked up on both of those when it was formed under Constantine so as not to confuse and alienate all those nice tithe paying pagans!