The prefix “co” here is in the sense of cooperating with. It does not mean “co-equal.” I’m a sense, we can all be co-redemtors by cooperating with Christ, by letting Christ live in us, and preaching the gospel. Mary has a unique role in salvation history in consenting to bearing the Incarnation, raising Jesus, following him, and pointing us to him, and this is recognized, among other titles, with the title co-redemptrix.
I am less familiar with mediatrix. As far as I understand it, it is pleasing to him to involve her in distributing all graces. Not that she’s the source, but as the Queen Mother she intercedes with prayer always for all Christians and people for their conversion. And she is an exemplary Christian, an arch-type of the Church, and among all the saints is the closest to God.