I'm only quoting what the Vatican said and the pope agreed with. I figured they would actually know actual Roman Catholic Theology - especially since the Author is DDF prefect Cardinal Víctor Manuel Fernández and authorized by the Pope.
Seems your issue is with them.
I actually read Mater Populi Fidelis and linked to the actual document. I encourage you to read what is says.
I finally was able to read it.
And yes, my issue is with cardinal Fernandez. He's the author of that book, 'Heal Me With Your Mouth' which was a treatise on kissing. His other book was even worse, and he tried to bury it. But he was a friend of pope Francis, and thus promoted way beyond the merits of his competence. He was given the position as head of the DDF in one of the strangest appointments ever, only after pope Francis got incredible heat for proposing someone even worse for the same job. Cardinal Fernandez was the author of the document 'Fiducia supplicans', which proposed that we can now bless married homosexuals, if only it is an impromptu blessing not of them as a couple but as two independent blessings in a non-liturgical and spontaneous act clearly different from an actual wedding. This was so wrong on the face of itthat most of the bishops in Africa, and at least a third of the bishops around the world were in formal dissent from this document and cardinal Fernandez had to back down. The document still stands, with a follower or two here and there but Fernandez has the egg on his face and the document has no power.
That Fernandez.
Now I sort of agree that speaking about Mary as co-redemptrix and as mediator of all graces is not terribly opportune. And that because most Protestants would go all apoplectic over mention of such things even if I went into the ten minute explanation of what is and is not involved with those terms. Most Protestants are invincibly ignorant on such matters and cannot endure hearing such things. So why bother? Many Protestants go apoplectic too about Mary as the mother of God, thinking that we have decided that she gave birth to the Father, and the Son, and the Holy Spirit. Discussions with those sorts of people is also inopportune. That is the extent of my marginal agreement with MPF.
Actually, much of the document appears to be from notes compiled years ago, before cardinal Fernandez and before pope Francis. That part, the historical discussion of past use of the terms, seems OK. Selective though, as there are some glaring omissions of past usage by popes and by Vatican II. That part leaves it understood that Mary as co-redemptrix or as mother of all graces are not in error. But that was the position of pope Benedict, who decided that is was not opportune to define those terms as dogma while he was pope. He did not rule against them, nor rule for them, but deferred the whole discussion.
This new document does not rule against believing in those two terms. It just says it is 'always inappropriate' to speak using those terms. I consider this to be a 'silencing' rather than a doctrinal statement. Fernandez could have made a doctrinal statement against those two titles of Mary. He didn't. He kind of left that an open question. But he expects us to bite our tongues nonetheless. That is a disciplinary matter. One that I was following even without his special encouragement to bite my tongue.
Cardinal Fernandez made a special point that this document, approved by pope Leo, is now part of the Magisterial teaching of the faith. Which is him implying that this is infallible teaching. Ha! It's a very fallible disciplinary matter, imprudent in that it raised an issue that very few Catholics knew anything about, and probably advanced the cause of Mary as co-redemptrix and as mother of all graces well beyond anything else that has been done in the last 100 years. In that sense an epic fail. Which is par for the course for things cardinal Fernandez touches.
Cardinal Fernandez is no expert on Catholic theology. His predecessors, cardinal Mueller and cardinal Ratzinger were such experts. I hope cardinal Mueller has some comments on MPF some time soon. In the mean time there have been plenty of comments on how inopportune and confused and pretentious MPF is. And I agree. Even while I wouldn't be touting Mary as co-redemptrix or mother of all graces anyway.