What is the difference between mediatrix and co-redemptrix?
I think they are similar and one can be applied to the other. However, just off the top of my head, I think co-redemptrix refers to the redemption which Jesus Christ wrought by his sacrifice on the cross for the salvation of the world. By his sacrifice on the cross and his suffering and death, Jesus redeemed us, he paid to God the penalty that was our due because of our sins as the prophet Isaiah says:
Yet it was our pain that he bore,
our sufferings he endured.
We thought of him as stricken,
struck down by God* and afflicted,
But he was pierced for our sins,
crushed for our iniquity.
He bore the punishment that makes us whole,
by his wounds we were healed. (53: 4-5)
Our Blessed Lady stood beneath the cross offering to the Father, her son, for the salvation of the world.
So, by the redemption, Jesus won for us or merited all the graces we need to get to heaven. Jesus united the sufferings of his mother Mary with his for the salvation of the world. Just as Eve played a part in the fall of mankind, so Mary, the new Eve, played a part in the redemption of mankind. Consequently, Mary has the title of co-redemptrix.
Mary, mediatrix of all graces, refers to the dispensing of the graces merited by the redemption of Jesus Christ of which Mary, as co-redemptrix played a subordinate part. All the graces we receive from God pass through her hands for she is our mother in the order of divine grace. She dispenses the graces merited by her son Jesus when she wills, as much as she wills, and to whom she wills. Since our Lady’s will is wholly united to God’s will, she does all according to God’s will.