I'm slightly mixed on this one. Mary gave birth to our Lord. Her acceptance of this virginal birth doesn't give her a "Co-" status in my opinion. I don't see how it can. We are saved through Jesus Christ dying on the Cross. Through Mary to our Lord. The great mystery is how us creations of God can not contemplate the amount of love God had for us to send his only begotten Son through Mary, to save us.. And then suffer greatly as man. It's actually quite mind boggling, when you think about it.
Had Our Lady not given her "Fiat", the Redeemer would not have come into the world. He did what He did through her and using her as an instrument. "Co-" does not inherently require there to be a footing of equality. Perhaps it's more an issue for English speakers. But in Latin, the prefix "co-" or "con-" simply derives from "cuм", the Latin word for "with", and who can deny that Our Lady was with Him throughout the Redemption and mediating the Redemption to the entire world? She is in fact the Mediatrix of all the graces from the Redemption, and as a result is a necessary cause, by God's will, of the application of the Redemption to souls, even if she isn't a sufficient cause. That role suffices for her to be a "Co-Redemptrix", but, the Church has refrained from defining the term solemnly or formally precisely because it could easily be misunderstood ... and of course the Prots would have a field day attacking the Church over it. Not that we care what Prots think per se, but stuff like that could present more obstacles for converting Prot heretics to the True Faith, since if even Catholics could misinterpret it, then you KNOW the Prots are going to misinterpret it.
One metaphor I use is the role of parents, who in a sense are co-creators for their children. God uses their free will to bring souls into the world, and bring them into existence. When a couple decide to have a child, then God creates a soul. So there's a direct COOPERATION, by God's will, in that act of creation. God could of course just create souls (and bodies) out of thin air, as He did with Adam, or, as Our Lord said, raise them up from stones, but He willed to allow parents to be involved in the co-creation of new human beings. It's the same way with Our Lady as Co-Redemptrix. By God's will, she was necessary for the Redemption to take place. God could have created Our Lord's human nature out of nothing, but made it contingent upon Our Lady's cooperation and her Fiat.
This also reminds of the dispute that the Nestorians had with Our Lady receiving the title "Mother of God." How can a human being be the "Mother of God"? Well, when the terms are rightly understood, the term is absolutely correct. Same thing with Co-Redemptrix. If it's rightly understood and defined, the term is theologically correct.