I’m not a linguist expert, but apparently your definition of the prefix “co-” is a little bit off the mark… here’s what I got from
this site.
*"The term “co-redemptrix” is properly translated “the woman with the redeemer” or more literally “she who buys back with [the redeemer].” The prefix “co” comes from the Latin term “cum” which means “with” and not “equal to.” Co-redemptrix therefore as applied to Mary refers to her exceptional cooperation with and under her divine son Jesus Christ, in the redemption of the human family, as manifested in Christian Scripture.
With Mary’s free and active “fiat” to the invitation of the angel Gabriel to become the mother of Jesus, “Be it done unto me according to your word” (Lk. 1:38), she uniquely cooperated with the work of redemption by giving the divine Redeemer his body, which was the very instrument of human redemption. “We have been sanctified by the offering of the body of Jesus Christ once for all” (Heb. 10:10), and the body of Jesus Christ is given to him through the free, active, and unique cooperation of the Virgin Mary. By virtue of giving flesh to the “Word made flesh” (Jn. 1:14), who in turn redeems humanity, the Virgin of Nazareth uniquely merits the title Co-redemptrix. In the words of the late Mother Teresa of Calcutta, “Of course Mary is the Co-redemptrix - she gave Jesus his body, and his body is what saved us.” [1]
The New Testament prophecy of Simeon in the temple also reveals the suffering, co-redemptive mission of Mary in direct union with her Redeemer son in their one unified work of redemption:
“Simeon blessed them, and said to Mary, his mother, 'Behold, this child is set for the fall and rise of many in Israel, and will be a sign of contradiction, and a sword shall pierce through your own soul, too” (Lk. 2:34-35).
But the climax of Mary’s role as Co-redemptrix under her divine son takes place at the foot of the Cross, where the total suffering of the mother’s heart is obediently united to the suffering of the Son’s heart in fulfillment of the Father’s plan of redemption (cf. Gal. 4:4). As the fruit of this redemptive suffering, Mary is given by the crucified Savior as the spiritual mother of all peoples,: “Woman, behold your son!’ Then he said to the disciple, 'behold, your mother!” (Jn. 19:27). As described by Pope John Paul II, Mary was “spiritually crucified with her crucified son” at Calvary, and “her role as Co-redemptrix did not cease with the glorification of her Son.” [2] Even after the accomplishment of the acquisition of the graces of redemption at Calvary, Mary’s co-redemptive role continues in the distribution of those saving graces to the hearts of humanity…"*
In Christ through Mary,
S4ntA.