I don't think anyone is arguing against your conclusion. However, when someone changes the last point from "Mary is the mother of God with us" to "Mary is the mother of God" that the problem starts. The Son of God has ALWAYS existed, agreed? He has ALWAYS been God, agreed? His incarnation, taking on human flesh, was not when he became God, but when God became a man. The consternation comes from referring to Mary as the Mother of God - which omits that qualifier you used, Mother of God with us (whom we know is Jesus Christ). Nevertheless, there is no escaping the fact that a "cult of Mary" and Mariolatry didn't exist until men started calling her Mother of God. That title glorifies her and was not necessary for establishing the Deity of Jesus Christ.