I agree that we’re all called, in the orthodox sense, to be co-redeemers, in the sense of praying for and suffering for others’ salvation. St. Paul refers to “completing in his body the afflictions of Christ” (Col 1:24) and suffering for the comfort and salvation also of the Corinthians (2 Cor 1:6 I think).
I think that supports the title of "Co-Redemptrix, as a number of Popes have called her.
The difference is that Mary is the worthy Mother of God. So, as a saint said, to comprehend the mother we must comprehend the Son. In other words, to understand the greatness of the Mother of God, a creature, we must understand how much grace she was given in order to be worthy to become the Mother of the Son of God.
God has exalted her to such an extent that it’s not possible for God to exalt a creature more, as Pope Pius IX says. Saint-doctors of the Church likewise tell us that it’s not possible for God to exalt a creature more than he does by making that creature His Mother.
This means that Mary suffered inexpressibly more than all creatures, through her great love for her Son and for each human person.
The title “Co-Redemptrix” also seems to re-affirm Christ’s humanity. That he wants our help in order to save other souls. Protestants believe this as well, implicitly, since they believe that we are saved through the writing of the Scriptures by humans, and through people hearing the Gospel preached, people praying for us, etc.
I feel so consoled and helped to be close to Christ, knowing that my mother is so exalted and yet is constantly close to me, helping make sure that I don’t move away from Jesus.