You're assuming the faith of the apostles, transmitted in this way, was not gradually corrupted over time. What if it was? The only way to be sure of what the apostles transmitted to the early Christians through word and deed is to look at their epistles, which fortunately seem to have been preserved with minimal alterations in most texts. That's the only thing we can be sure of, I think. At least it's the only thing I'm sure of.
What if the Fathers were wrong? What if they had a hand in corrupting apostolic teaching? Look at any protestant study Bible, and you'll see there are other possible interpretations of the passage. It may be a literal vision of a future temple, it may be symbolic of the Church (whatever that truly is). The East gate may remain shut signifying that once God had interfered, he would never leave. It may be shut to prevent or signify the end of the solar worship in the temple described earlier in the book of Ezekiel.
What if it wasn't transmitted faithfully? What if things were added, like the perpetual virginity, the dormition, the intersession of the saints in heaven, monastic traditions, and so forth, all clearly built up over time? If the Church is that which faithfully preserves the teaching of the Apostles, and if Orthodox tradition contains later corruptions and merely human teachings, then the Orthodox Church is not the Church, and has no authority, any more than the Baptists or the Pentecostals. In the end, only the Lord knows who is His.
If, as you suggest, the orthodox Christian faith delivered once and for all to the saints (Jude 3), became corrupted over time, then the consensus of the Fathers and the decisions of Ecumenical Councils regarding the ever-virginity of the Theotokos are not trustworthy.
However, the Fathers such as St. Irenaeus, St. Vincent of Lerins and others certainly claimed that the orthodoxy of the Church was due to her faithfully transmitting the Apostolic faith from generation to generation. St. Irenaeus said the following:
The Church, which has spread everywhere, even to the ends of the earth, received the faith from the apostles and their disciples....The Church, spread throughout the whole world, received this preaching and this faith and now preserves it carefully, dwelling as it were in one house. Having one soul and one heart, the Church holds this faith, preaches and teaches it consistently as though by a single voice. For though there are different languages, there is but one tradition.The faith and the tradition of the churches founded in Germany are no different from those founded among the Spanish and the Celts, in the East, in Egypt, in Libya and elsewhere in the Mediterranean world. Just as God’s creature, the sun, is one and the same the world over, so also does the Church’s preaching shine everywhere to enlighten all men who want to come to a knowledge of the truth. Against Heresies
So the Church certainly believed that changing the Deposit of the Faith meant corrupting it. In many cases, they literally preferred to die as martyrs rather than change the faith. So it does not seem plausible that the Church would consciously change the faith.
Moreover, the historical evidence is strong that the perpetual virginity was believed early in the Church (we have evidence from 150 AD), widespread (basically all the Fathers) and in every part of Christiandom. Opposition to the belief only arose in the late 4th century by two writers, who were strongly condemned. Ecumenical Councils, clearly at the 5th Council, demonstrated that the Church universal accepted the ever-virginity of the Theotokos. There was no remnant of the Church that believed she was not ever-virgin. 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?
In addition, Christ's promises to the Church render the idea that it quickly entered into widespread heresy inconceivable. He said that He was the head of the Church, (Eph 4:15) and that the gates of Hades would not prevail against the Church (Matt. 16:18). He promised to send a Helper, the Spirit of Truth (John 14:16-17). He said
I have been given all authority in heaven and on earth. Go, then, to all peoples everywhere and make them my disciples: baptize them in the name of the Father, the Son, and the Holy Spirit, and teach them to obey everything I have commanded you. And I will be with you always, to the end of the age." Matthew 28:18-20
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?
Similarly, and more practically, why would you want to be a Christian, if with such a simple thing--the ever-virginity of Theotokos--the Church could not even keep that strait and true for one generation? Why would you have any confidence that the Church correctly understood and transmitted more important doctrines such as the Trinity?
Finally, your line of questioning begs the question of your own epistemology. Why should we trust your (or my) private interpretation? You say, regarding Scripture "That's the only thing we can be sure of, I think. At least it's the only thing I'm sure of."
If Scripture is the only thing you can be sure of, than you cannot be sure of your statement "That's the only thing we can be sure of"
, since it is not Scripture. Your position is self-defeating and you are thus in error. You are concerned that individual Fathers err, but so do you.
Yes, individual Fathers can err, but the Spirit guides the Church to truth. We can be confident of that truth when the Church speaks in unison, as it has regarding the ever-virginity. Scripture does not interpret itself; it needs a formal authority to do so. Why are your interpretations of Scripture better than that which the Church universal believed "everywhere, always and by everyone".
Moreover, after casting doubt on the reliability of the Church's decisions, you exalt the authority of Scripture. Do you believe the New Testament Canon is closed? If so, why do you accept the Scriptures that a "corrupted" Church selected from amongst many more possible documents? Do you think Hebrews is Scripture? Who wrote it? Why is its inclusion in the Bible trustworthy? Here is a list of Christian documents floating around during the time of the Early Church: http://www.earlychristianwritings.com/
The existing NT Canon says nothing about these books, so according to that which you are "sure of", we have no means to select the NT Canon. How do you do it without the Church and why is your decision normative?
For the reasons I have outlined, we can conclude that the Church did not fall into universal corruption over time, and thus the consensus of the Fathers and the Ecumenical Councils are trustworthy witnesses of the doctrine, including the doctrine of the ever-virginity of the Theotokos.