Mary cooperated with Jesus’ work of salvation on behalf of sinners. Of course, even today she cooperates in the distribution of the fruits of redemption. Not a single grace comes to us but through her intercession. However, she also cooperated at the foot of the cross in the work of the redemption itself.
This is established teaching.
Cardinal Pacelli, in a sermon about 80 years ago, Dec. 7:
After all, the application of the merits of Christ constitutes, together with their acquisition, a single, complete work: that of salvation. It was fitting that Mary should co-operate equally in the two phases of the same work; the unity of the divine plan demands it. Dec 7, 1937
Later, as Pope:
“As Christ the new Adam must be called a king not merely because he is Son of God but also because he is our Redeemer, so analogously the Most Blessed Virgin is queen not only because she is Mother of God but also because as the new Eve she was associated with the new Adam.” See DS 3913ff
Who, then, were the detractors against this Marian Privilege?
First, Heinrich Lennerz, S.J. Then, Karl Rahner, S.J. And also the early, and rather late, Ratzinger (who embraced the substance as pope). Also M. de la Taille; Billot; et alia.