While I understand what you are saying, I think there is a lot more to it. The complication for me is not in understanding how she cooperated with God’s salvation plan. I get it. But believe me, when someone tells me that Mary is the Mediatrix of ALL graces, I have a problem. Mediatrix of ALL grace can only be narrowly understood to mean that “All Grace” comes through Mary because Jesus came through Mary. To me, it is just to complicated to give a title like “Co-Redemptrix” or “Co-Mediatrix” and then have to give a big long justification as to why.
We are all “co” redeemers and “co” mediators when we help bring people to Christ. “Co” in this context simply means means “with” and so it is easy to say we are all working with Christ for the salvation of souls without diminishing His role in salvation. James 5:19 says “My brothers, if anyone among you should stray from the truth and someone bring him back, he should know that whoever brings back a sinner from the error of his way **will save his soul from death **and will cover a multitude of sins.” This does not mean we are the Savior, but as you can see from this scripture we can “save” a soul when we bring it back from error. Likewise, we are all “co” mediators when we pray for our brethren. None of us has as significant of a role as Mary, who said yes to God and brought forth our Savior. Her “co” role is clearly greater than any of ours. YET, I think the proposed 5th Marian dogma goes too far in elevating Our Lady, particularly when it says she is mediatrix of all graces. Personally, I do not believe it is a title she would seek because it places a lot of attention on her and not Her Son, and because it divides the Church.
Lumen Gentium did say “Therefore the Blessed Virgin is invoked by the Church under the titles of Advocate, Auxiliatrix, Adjutrix, and Mediatrix.(16*) This, however, is to be so understood that it neither takes away from nor adds anything to the dignity and efficaciousness of Christ the one Mediator.” However, those titles are not the same as “Co-Redemptrix and Co-Mediatrix” which if taken out of context could imply that we believe that Mary’s role is equal with the role of her Son. I think it is important to remember paragraph 67 as well: