Who would be most against the idea of Co-Redemptrix?
Mary, as in her "Magnificat," she seeks only to glorify her Son, Jesus, through the Eucharist and confession.
Mary's role is great, indeed, but it is one of humble, obedient submission — completely opposite of Satan. Mary's connection is vertical, not horizontal. Through the incarnation, Mary personifies the greatest virtue, humility, because she personifies grace (Luke 1:28, 30; John 1:14).
In Luke 1:28 of the Catholic Bibles, we have: "Hail (Mary) 'full' of Grace, the Lord is with thee. Blessed art thou among women.' " Protestant Bibles often reduce this to merely "highly flavored."
John 1:14 associates the Word (Jesus Christ) as being "full of grace." For Mary to be "given" this same phrase is very significant. Mary is not to be worshipped — but Most Holy Triune God "gave" her great grace to be the human tabernacle for her Son, Jesus Christ. She is queen of all saints and the "intercessory prayer" mentioned in Apocalypse or Revelations 5:8 and 8:3. Sacred tradition gives another title: "Mother of the church" (also based scripturally in Jesus giving His mother to John at the cross).
Hannah prefigures Mary in the Old Testament. Hannah means "grace." Difference? Mary is grace. If there is a hole in the ground, I — especially — and Hannah will fall into the hole. Mary is entirely protected beforehand from falling, not mainly by her own efforts, but because Most Holy Triune God must have the "purest" vessel in the incarnation of Jesus Christ, who is God. Christ's crucified flesh necessary for salvation is from Mary.
The "Magnificat" connects Mary and Hannah in Luke 1:46-55, 1 Kings or 1 Samuel 2:1-10, and Psalms 112 (113).
Essentially, who is praised? "Most Holy Triune God." He has no co-pilots, nor is the author of confusion. Grace is more fitting than Co-Redemptrix for the 5th Marian Dogma.