What will happen by saying it like this is that those who already misunderstand the teaching of these will say that the Church has changed her teaching, when she has not.
Protestants misunderstand what we mean and the theology behind Mary being co-redemptrix. It comes from the Early Church Fathers who pointed out that Mary is the “New Eve” (see Justin Martyr, St. Irenaeus of Lyons, Cyprian, and St. Ambrose, who all described Mary as the New Eve).
Essentially it is this, Where Adam (and Eve), by their disobedience to God, brought sin and death into our world, Jesus (and Mary), by their obedience to God, brought reconciliation and life into our world.
The prefix co, does not mean equal to. Eve played a part in Adam’s sin, but Adam is responsible for the fall. Mary played a part in the redemption of the world, but Jesus is responsible for rising and conquering death.
Jesus is divine and Mary is not divine.
Another way that Mary is the New Eve is that when God made Eve, she was called a “helper” (Genesis 2:18) and Mary refers to herself as the handmaid, or “helper”, of the Lord (Luke 1:38).
Lastly, when you read Genesis 3:15, it says God would put enmity between the woman and the serpent and between her Son and the serpent’s offspring. May is often called “woman” in the New Testament. Not as a derogatory term, but rather a callback to this promise.
Many Protestants (not so much the so-called Reformers) put enmity between Mary and Jesus. Mary is on Team Jesus. If you are not on Team that Mary is on, then you must be on Team Serpent. Don’t be on Team Serpent and don’t put Mary as the Team Captain. Jesus is our Team Captain. I hope this makes sense.
As far as co-mediator goes, I want to parrot what @Literalman said, there is one meditator between God and man: Christ Jesus (1 Timothy 2:5).
We believe in the Communion of Saints. And we believe that those who die in this life, but are alive in Christ are not separated from the body. In fact, we believe they are more alive than Christians are here on Earth. And we ask for their Prayers. Mary, the Theotokos, “Mother of God,” we ask for her Prayers.
At the foot of the Cross, Jesus looked at His beloved and then to Hid Mother, and said, “Woman, behold your Son” and to His disciple, “Behold your Mother” (John 19:26-27). In a spiritual sense, Mary become our blessed Mother.
Also, Christ is King! And Mary, as His Mother is Queen. And we see in the Old Testament that Queens played a special role in the Kingdom. They held a place of honor. And thus, we honor Mary.
Again, I hope this makes sense.
Have a good night : )