"Mary was a good person and had a sinful nature like all of us." - the same could be said of Stephen.
Stephen "being full of grace"
Acts 6:8 And Stephen, full of grace and power, was performing great wonders and signs among the people.
And in Stephen's case it is not just "full of grace" but "full of grace AND power"!
And what is true of Christ is that He was incarnated -- not procreated. Which is true of Christ alone - not any other human
I would say that Protestants did the right thing in drawing a line between them and the Catholic Church. One must reform traditions that create non-biblical guidelines to living in Faith.
I know this is a very old post but a reply to it was sent to my feed so I must have commented on it at some point. Rather than try to search for it, I thought I would comment again. Likely this time with more diplomacy.
The logic you have put forward is very clear. However, I believe that the difference between St Stephen and the virgin mother is somewhat apparent from my point of view (obviously coming from a slightly different theology).
The Church declares that the Virgin Mother was sinless because an incarnation of the Almighty ruler of the universe could not choose to be brought forth from an unclean vessel. The original Church has made this declaration. It is important to remember that the Holy Greek, Roman, Jerusalem, Hippo, and many other Churches were unified in doctrine. I preface that by saying that our fist Apostolic and Holy Church was founded close to 400 years before the New Testament Cannon was approved. There were many Apostolic writings in circulation and writings from the Magisterium before that was decided.
That obviously means that the Church was teaching it's theology hundreds of years before they decided what books would be included in the New Testament. Also, it might not have even been made manifest without Emperor Constantine's direct order to summarize the Gospel message. I think they did a good job documenting the most popular and well-known teachings of Jesus and his disciples. However, though The Bible has became our most reliable manuscript of faith, it does not document every word that was taught in those first 400 years. It still remains the best summary of faith but The Church existed to create the Bible, The Bible didn't creat the Church.
There are many other historical writings in the vaults of the Vatican that also have teaching within them and much was taught by word of mouth. In fact Jesus never wrote a word of scripture himself. So while the Bible is divinely inspired, it is not literally divinely written. I am sure that sounds blasphemous to a Protestant as the entirety of their religion is contained within the Bible.
I'm not trying to excuse any wrong teaching or trying to convert anyone to Catholic theology. I am merely trying to explain how a teaching like Mary being sinless came to be. Regardless of what is in or not in the Bible, my understanding of the Immaculate Conception is that God would not choose a sinful vessel for his entrance into humanity and I believe that it one reason why Christ was sinless.
I do not disagree that Mary also had a sinful nature, I just don't believe that she acted on that nature. I also believe it is one of the reasons that she was chosen to be the vessel. That and her extraordinary faith. She did her job and funded Christ as a baby, fed and nurtured him in a perfect way so he had the full understanding of who he was and what he was there to do, not to discount Father God, and the Holy Spirit from this task as well.