Okay I will believe your position that Mary does not communicate all graces which are necessary for salvation when you show me which other human Christ (the source of all grace) came through during the Incarnation.
The Incarnation occurs in two ways: 1. historically, at the unique moment of that conception in Mary’s womb, and 2. daily, in each baptized Catholic “incarnating” the word by word, deed, action, and love.
Why must we assume that, because Christ became incarnate through Mary, thus Mary stands before Christ at all times, the only door to Christ and the only portal through which His grace comes to us? The Lord is incarnate, and He is no longer in the womb or the mother’s arms - He is his own person, alive, reigning, ruling, and commanding every legion of the angels. The Incarnation lasts, and does not need to be renewed by Mary every day.
Also, yes St. Paul is right that there is only one mediator with the Father; however, St. Paul said nothing about there not being a mediator with the Son, so please don’t take St. Pauls words out of context.