The freedom of the Blessed Virgin

By her participation in redemption [the Blessed Virgin] had filled heaven with the saved and had emptied hell of those who would have been condemned except for her. It was her assent to the word and will of God that had made the incarnation and therefore the redemption possible. [Pelikan, The Growth of Medieval Theology, 166]

It's not that Mary has saved us; rather, through her the Redeemer descended to us (as St. Bernard says; cf. Pelikan, 165). She is mediatrix in the sense that Christ comes to us through her. It's important to be clear that her assent was not something compelled; as in everything else and like all men, she was a moral agent. God did no violence to her will, just as he did none to Eve's.