Most protestants applaud the Catholic Church for sticking to Conservative principle on many things. The core beliefs of Catholics make them brothers in Christ. But Where most Protestants scratch their heads is this nearly religion of Mary that has been created. I am OK if you want to pray for intercession or if you want to display artwork of the virgin Mary. It is Mary is the co-Redeemer that goes to the point of conflicting with the Bible. The Catholic Church takes their Mary doctrine way too far, and even if you want to argue that the official Church position does not cross the line, the doctrine takes many Catholics to the point where they do cross the line into worshiping Mary. And that is how many non-Catholics see it.
Second, Mary is not, by any stretch, the primary cause of our Redemption. Christ alone is. Hers is a secondary adjunct to our redemption, and the aforementioned Luke 2:35 passage is the best simply human fulfillment of what St. Paul is talking about in Colossians 1:24. Nothing we aver about Mary is any more contradictory to the facts of Christ's redemption of the human race than what St. Paul says in Colossians. No outcry is made about his statement, likewise, none should be made about the proper understanding of Mary's role.
Third, none of this has anything to do with the worship of Mary. She is not divine. She is merely - and solely and completely! - human. To worship Mary is idolatry, pure and simple. To give her the honor, love, respect and veneration she is due is not "worship."
Indeed, on a lesser scale, the Early Church similarly honored, loved and venerated the martyrs, upon whose graves the Holy Sacrifice of the Mass was said, a practice documented to the early 2nd Century (and likely extending back earlier). If the practice of veneration of the Saints and Martyrs is inherently "wrong," then take it up with the earliest Church. But, if they were wrong, then the fact that such "wrong" practices have continued from their day to ours demonstrates a wretched lack of providential care for His Church by God, and provides evidence that the whole of the Christian Faith is a sham and a fraud. Consider the implications of that a while before casting stones at the veneration of the Saints! It is the perfect logical correlary to what St. Paul says in 1 Corinthians 15:14.
The problem with Protestant objections to Catholic beliefs and practices is that they are based on "novel" doctrines. Every one of them dates no further back than the 16th Century; many aren't even half that old. There is therefore, little logical point in calling various Catholic practices "unbiblical" when the Apostolic Era and its immediate aftermath are shown to harbor and nurture them again and again. The authority for them rests in their ancient lineage, and it is cheeky, at best, to object to them on the basis of understandings that can only trace their origins 1/4 of the way back to the beginning. Unless, of course, God did not exercise providential care for His Church, which He promised would both withstand the gates of Hell (Matthew 16:18) and be directly guided by God to the end of the age (Matthew 28:20) to be "the pillar and bulwark of the Truth" (1 Timothy 3:15).
But, again, if God either did not, or could not, exercise the providential care for His Church that He promised, we are - all of us, Catholics, Protestants and Orthodox alike! - wasting our time in a false religion! I prefer to think that he meant what He said, and the Church He founded has not, can not and will not stray from the Deposit of Faith in any matter. Because He promised!
You know, I disagree that co-Redeemer is unbiblical, -- more on that in a moment -- but first, what you say here is remarkable because it is a reasonable position. When Catholicism is denied because of veneration of saints or the Holy Images (that is not "artwork", tsk tsk), that is plain absurd, but it is fair to point out that devotion to Mary seems out of proportion at times. Thank you for the reasoned tone of your post.
The dogmatic position of the Church is that Mary is a central figure of the Incarnation and has a mystical connection with the Catholic Church. The former is, of course, because of her uniquely intimate and uniquely miraculous involvement in the Incarnation. The latter is based on her adopting the "disciple Jesus loved" at the foot of the Cross, as well as, of course on her mystical ability to "magnify the Lord" so that through her maternal suffering "out of many hearts, thoughts may be revealed".
Co-redeemer would be heretical is this relationship between Christ and the Church, -- based, in truth, on Mary's admonition to us "do what He tells you", -- is allowed to become a relationship of equals. I am not aware of anyone's Marian devotion to take on that tilt, but in theory it is a danger of the co-redeemer title. If it is ever recognized officially, solid catechism on that will be required.
If "co-Redeemer" is understood, correctly, as Mary in her scripturally detailed role in the Economy of Redemption, then, of course, it is inobjectionable. It also has a spiritual benefit to us pedagogically, as it is useful to meditate on the Gospel through the eye of Mary, -- a human being like us, but most intimately involved in the mystery of our salvation. This is the usefulness of the Rosary prayer to all Christians.