I have tried to explain that I am disagreeing with what I think is being put forth, but its hard to quote exact quotes because its a little hard to understand exactly what is proposed. I can tell you I disagree with Dr. Miravalle and Msgr. Calkins, for example, but maybe you are suggesting something other than what they say. Between them they wrote much of what EWTN puts up on the topic, but most of what they write is apologetic, not explanatory.
That said, Mirvalle says that the doctrine means that Mary has an active role in salvation. I object to this statement. Mary had an active role in salvation through her role in the Incarnation. She now advocates for salvation. She has no active role in the salvation of individuals.
Similarly, I object to the statement that Mary “with Christ redeemed mankind.” I realize that Benedict XV said this, but I disagree. Christ redeemed mankind, not Christ and Mary. Mary assisted by cooperating with God’s plan. All of us have a duty to do the same, none of us will do so as well or in as important a way. But that cooperation is not redeeming mankind. Only Christ did that.
I object to the statement (by Calkins and others) that Mary distributes the grace that flows from the Redemption. Christ made clear, as did John, Paul and the evangelists, that grace comes to man directly from God. Its not even really accurate to say it “flows” although that is the common word used. The Spirit dwells within us, and causes God’s grace to well up within us. There is no intermediary distributing graces.
There are more objections, some of which I have already noted. But, I would point out that John Paul II, who was deeply devoted to Mary, ask a Pontifical Commission to look at precisely this issue in 1996. That commission found the meaning of both Mediatrix and Co-Redemptrix to be ambiguous, and that references to similar titles by past Popes were not significant or definitive. The commission noted that this had been looked at three times by commissions and that each has rejected declaring the titles. They also noted that because the titles are not well defined, if they were declared now theologians would have to figure out what that meant after the fact, and that in the meantime the confusion would be damaging to ecumenism. All of these points have been made on this thread.
Declaration of the Theological Commission of the Pontifical International Marian Academy