The English phrase has roots entirely in Latin, Petronski, and so splitting hairs between one or the other does not alter the meaning.
But, just running with this for a second, if in fact "co-" merely means "with," as several have stated, then why the feminine form of Redeemer? It makes no sense, linguistically, unless the intended meaning is "with the female Redeemer." Clearly, the intended meaning is not that.
Therefore, "co-redemptrix" means the female partner in Redemption, which is the meaning that you and others are at pains to avoid, apparently due to the faint recognition that such a belief is absolutely contrary to the Bible, in which Redemption is through Jesus Christ alone.
It means what it's proponents say it means.
You do not get to redefine it.