Let me see if I have the sequence of events straight:
1. Calvin believed Catholic Marian beliefs including Mary's perpetual virginity.
2. Calvin broke from the Catholic Church.
3. Others followed Calvin and became Calvinists.
4. Calvin continued to preach about his Marian beliefs including perpetual virginity. In fact, he never altered these beliefs.
5. At some undetermined later date, Calvinists began to question Calvin's Marian beliefs and rejected them.
So, when you say that Calvinists "needed to get as far away from Rome -and the suffusion of God in their myriad superstitious accretions- as they could," you're not really being accurate. These people had already left the Catholic Church, the Calvinists who rejected Calvin's Marian beliefs centuries later had probably never been Catholics in the first place. By rejecting Calvin's Marian beliefs, it was Calvin they were leaving, not Rome.
Nowhere does Calvin affirm the noxious belief in the Immaculate Conception of Mary which states Mary was herself conceived without sin and did not suffer from Original Sin; the non-birth birth of Jesus Christ whereby Jesus did not disturb Mary's physical virginity; or the Assumption of Mary which dares to say another human being other than Christ rose bodily into heaven -- all concoctions of fanciful mischief and damning error.
When I refer, though I didn't in my post to you of this mysterious and as I see it very unhealthy and misogynistic need to pristinate Mary, I'm referring to the dogma of the Immaculate Conception, that's irrelevant as far as your post to me is concerned, but I thought I'd take this opportunity to elaborate on that in this post to you.
So let me get on to and with the Calvin/Luther part of it. Luther may have affirmed her perpetual virginity and Calvin too, and IIRC what Calvin really said when explaining himself on the issue was that one way or another it's idle speculation, and that it serves no purpose, and I really couldn't care less whether she had conjugal relations or not. If she did it takes nothing away from her, but instead makes her a wife who loved her husband.
The crux of the matter is that (IIRC) neither Calvin nor Luther said a person's salvation was contingent, even marginally so, on holding that belief.
I don't expect you to understand, but I thank God for the Reformation. I thank God for the Reformers because they gave me a refuge from a Rome that I consider to have very harmful propensities. That is the Rome I know, not on paper, but in fact and deed. They are who they introduced themselves to me as, and all the darkness that carries with it, and they always will be. They have only themselves to blame.
I would say may God have mercy on them, but I'll worry about God having mercy on me for my own sins first.