I think you overcomplicate the matter. As I read the Bible, there are three components to salvation: believe in Jesus, repent, be born again. Admittedly Catholics pray to Mary (and various other saints) and I think they are wrong to do so. All of us, Protestant and Catholic, do many things which, taken alone, might cast our salvation into doubt according to someone. But do those things invalidate our salvation if we’ve met the three criterion? No, because the blood of Jesus covers those sins and errors. So, praying or not praying to Mary won’t invalidate your salvation; believing or not believing the bread becomes the literal body of Christ won’t invalidate your salvation; drinking grape juice instead of wine at communion won’t invalidate your salvation. Do you get my point? These matters are principles worked out by the various religious groups for reasons that seem right to them, but they are NOT the basis for your salvation.
Yes, the eternal virginity of Mary is a “doctrine” without a purpose.
It has no effect on believing on Jesus for salvation.
If it is SO important, why does scripture imply that Mary DID have other children?
No one taking the scripture at face value would see a perpetual virginity state for Mary.