OK, Clemente, I'm going to attempt to answer your questions. bare in mind that I'm not an ardent Protestant, though perhaps my previous posts might have given that suggestion. Truth is, I'm thinking about possibly joining the Orthodox Church, which is why my monicker says Orthodox Inquirer, and not Protestant or something like that. But I have a lot to figure out and a lot of ideas to sort through, and I can see an argument for and against issues like the ones we're talking about here. I'm a long way from knowing if the Orthodox Church is trustworthy and 'safe', so bare with me. I will attempt to impart my thoughts on the questions you have asked, and will appreciate honest feedback.
Why is it historically plausible to think an entire Church would enter quickly into heresy by accepting an error regarding the Theotokos without leaving a paper trail of opposition?
If anything the ancient Church believed about the Theotokos was heresy, it wasn't the ever-virginity per say, but what it and other ideas gradually led to over the course of centuries. I'm talking about the way, as it sometimes seems to me, that the Mother of God was exalted to near-goddess stature. I've talked about this in my Theotokos thread, and I won't derail the topic. But assuming the veneration of the Theotokos is
excessive to the point of heresy, I would posit that, it escaped notice for two main reasons:
A, Because it crept in rather gradually compared to other heresies which seemed flare up over night, so gradually that few if any noticed the changes in teaching and emphasis from the grace of Christ to the graces of His mother. The teaching of the perpetual virginity, which is attested to in the mid-second century, would only have been one step in the long process I'm hypothesizing.
B, Because devotion to and veneration of Mary, along with belief in the perpetual virginity, were useful in combating other heresies. So when Nestorius tried to butcher the authentic doctrine of the Incarnation, he wanted to call Mary the Christotokos. The Church, rightly I think, responded by affirming Mary's identity as the Theotokos. And the Perpetual Virginity, even if unnecessary, turned out to be very useful for affirming the teaching. If God were in her womb, surely she could not be contaminated by the seed of sexual intercourse. She was like the ark that could not be touched, and thus she remained a virgin. This also fits with conceptions some of the Fathers had about sexuality, that it was a result of the fall and not a truly natural human faculty, less than perfectly honorable and not to be chosen, even in marriage, above the great and awesome glories of celibacy, which was seen as the 'angelic state'. These are teachings I'm not naturally inclined to agree with, for the same reason I'm uncertain about the rest. I don't see these ideas in the scriptures.
If the universal Church fell into heresy regarding the ever-virginity, then you have one of two choices: either the the Holy Spirit did not lead the Church to truth, in which case, Jesus lied, or the Holy Spirit was impotent and therefore somehow less than divine (in which case you are rejecting the Trinity) or somehow ineffectual. Which do you believe?
Don't forget the Scripture promises there will be heresies, even at the heart of the Church.
"Therefore take heed to yourselves and to all the flock, among which the Holy Spirit has made you overseers, to shepherd the church of God which He purchased with His own blood. For I know this, that after my departure savage wolves will come in among you, not sparing the flock. Also from among yourselves men will rise up, speaking perverse things, to draw away the disciples after themselves. Therefore watch, and remember that for three years I did not cease to warn everyone night and day with tears."
Acts 20; 28-31.
Paul also says this:
"And He Himself gave some to be apostles, some prophets, some evangelists, and some pastors and teachers, for the equipping of the saints for the work of ministry, for the edifying of the body of Christ, till we all come to the unity of the faith and of the knowledge of the Son of God, to a perfect man, to the measure of the stature of the fullness of Christ; that we should no longer be children, tossed to and fro and carried about with every wind of doctrine, by the trickery of men, in the cunning craftiness of deceitful plotting, but, speaking the truth in love, may grow up in all things into Him who is the head, even Christ"
Ephesians 4; 11-15
He said this indicating, I think, that the Spirit of God would eventually lead the Church out of errors and bring her to perfection, but no one in the scripture tells us that this process would be swift and easy going,