You wrote:
“Protestants believe to assign a sinless nature to anyone other than Jesus Christ is a grave error.”
And Protestants in a similar fashion simply believe whatever they choose.
“Nowhere in Scripture is Mary referred to as anything other than a devoted mother of the man, Jesus Christ.”
Nowhere in Scripture does it say that something must be explicitly in scripture to be true.
“Christ even corrects those who sought to assign her some unique position...”
Nonsense. All Jesus was saying was that it was more important to be part of Jesus’ spiritual family than his earthly family. Mary was a member of both families. She is Christ’s mother, and she is the best example of someone who hears and keeps the word of the Lord (Luke 1:38, 45).
Actually your church is in a double bind, because it teaches a contradiction.
Sin is the transgression of the law.
So to have a sinless nature and life would mean that one would have to keep the law perfectly
See what your church teaches on that
578 Jesus, Israel's Messiah and therefore the greatest in the kingdom of heaven, was to fulfill the Law by keeping it in its all embracing detail - according to his own words, down to "the least of these commandments".330 He is in fact the only one who could keep it perfectly.,331 On their own admission the Jews were never able to observe the Law in its entirety without violating the least of its precepts.332 This is why every year on the Day of Atonement the children of Israel ask God's forgiveness for their transgressions of the Law. The Law indeed makes up one inseparable whole, and St. James recalls, "Whoever keeps the whole law but fails in one point has become guilty of all of it."333