"Co" in the sense of cooperation, not co-president.
IOW, Mary's cooperation with the Will of God brought our Redeemer and Mediator into the world. In this sense, she is our Co-Redemptrix and Co-Mediatrix. She is not divine.
The terms are ambiguous and confusion regarding their meaning is understandable.
It is not that "Mother of God" may not ever be technically allowed in an defining expression, as the Holy Spirit is careful to provide regarding Israel being the progenitor of Christ, "of whom as concerning the flesh Christ came, who is over all, God blessed for ever. Amen," (Romans 9:5) but the problem is that this is not how Mother of God is expressed, but that even the more accurate title God-bearer (Theotokos) is shunned in preference to a term which most naturally denotes ontological oneness, that Mary contributed to the deity of Christ.
This is not the language of Scripture any more than "Co-Redemptrix" is, and even Ratzinger's objection to Co-Redemptrix also applies here.
when asked in an interview in 2000 whether the Church would go along with the desire to solemnly define Mary as Co-redemptrix, then-Cardinal Ratzinger responded that “the response of the Congregation for the Doctrine of the Faith, is, broadly, that what is signified by this is already better expressed in other titles of Mary, while the formula “Co-redemptrix” departs to too great an extent from the language of Scripture and of the Fathers and therefore gives rise to misunderstandings” (53).
He went on to say that, “Everything comes from Him [Christ], as their Latter to the Ephesians and the Letter to the Colossians, in particular, tell us; Mary, too, is everything she is through Him. The word “Co-redemptrix” would obscure this origin. A correct intention being expressed in the wrong way. “For matters of faith, continuity of terminology with the language of Scripture and that of the Fathers is itself an essential element; it is improper simply to manipulate language” (God and the world: believing and living in our time, by Pope Benedict XVI, Peter Seewald, Ignatius Press, San Francisco, 2000, p. 306
And if "Mother of God" and other titles are to be used by way of extrapolation, then Mary's mother can be called the grandmother of God, and so forth all the way back to Eve. But the unqualified use of Mother of God is used as part of the Catholic hyper exaltation of Mary as an almost almighty demigoddess who cannot be praised to much, without ever crossing the ambiguous line into Latria. Which hyper exaltation of any mortal is never seen in Scripture, and we are warned not to think of men above that which is written.