Fr Garrigou-Lagrange explains why the Blessed Virgin's universal mediation is theologically certain (sent. certa), and not merely a pious and probable opinion on account of its organic connection with established theological principles. The Mother of the Saviour, Ch. 3, a. 3.
Mary's work Glorifies Gods work in the Highest. She is the Queen with an active role not a tube.
53. No human person — not even the Apostles or the Blessed Virgin — can act as a universal dispenser of grace. Only God can bestow grace,[135] and he does so through the humanity of Christ[136] since “the man Christ possessed supreme fullness of grace, as being the only-begotten of the Father.”
We must understand Mary’s mediation not as a complementary aid that would enable God to work fully, with greater richness, and more beauty; instead, her mediation must be understood in such a way that “it neither takes away anything from nor adds anything to the dignity and efficacy of Christ, the one Mediator.”