(1) his influence on the development of the Canon, which is always an ecclesial, not an individual, determination; and
(2)his evidence of what the faithful believed in the early centuries of Christianity.
Your guess about the background of this belief is exactly wrong, historically speaking. The belief in the ever-virginity of Mary was the older universal belief; and the notion that she gave birth to other children was, at Jerome's time (fourth century) a shocking innovation.
And it’s not just the Roman church, if by that you mean the Latin West. Mary is honored as Ever-Virgin in all the earliest Apostolic patriarchal churches -— Jerusalem, Antioch, Rome, Constantinople, Alexandria -— as well as churches historically more independent of their direct influence, such as the Armenian, Assyrian, Coptic and Ethiopian churches.
These held the doctrine of "ever-virginity" centuries before the Sixth Ecumenical Council, which finally officially defined it as a doctrine. The 3th century Armenians, for instance, who were not even part of the Roman Empire, did not adopt this doctrine "because" a council in Cobstantinople(which they did not attend) defined it officially in the 7th century. Think.
In fact, the broader geographically, and deeper historically you go, the more it looks like “Ever-Virgin” is the authentic belief from the Apostolic Era, and divergence from that was an innovation. An unpersuasive one, too. It didn’t get any traction with any Christian churches anywhere, Roman or not. After Jerome refuted Helvetius' "innovation," controversy didn't arise again until millennium later.
It is pretty enlightening to look into the history, which is a rich field for the action of Divine Providence.
At Jesus' time when he appeared to be just another man--albeit and unusual one--that she did not give birth to other children would have been a shocking innovation. If she hadn't, and being so fundamental to doctrine, the Scriptures would have shown that she hadn't.
But, let's face it--it doesn't. To the contrary, it clearly says that she did, and other Scripture supports the idea the the general epistle of James (Iakob) and the letter of Jude were written by two of His brothers.
I'll tell you what.
The “Ever-Virgin” for YOU guys turned into the "Ever Frustrated" for ME!!!
--St. Joseph(Charlie Brown got off easy!!)
And who was one of other scholars who rejected or doubted apocryphal books as being Scripture proper (seen even in Trent), and thus Rome does not follow his judgment in the very thing you invoke him for.
(2)his evidence of what the faithful believed in the early centuries of Christianity.
And who testified that “The presbyter is the same as the bishop,...And this is not my private opinion, it is that of Scripture. If you doubt that bishop and presbyter are the same, that the first word is one of function, and the second one of age, read the epistle of the Apostle to the Philippians....it is fitting that the bishops, on their side, do not forget that if they are set over the presbyters, it is the result of tradition, and not by the fact of a particular institution of the Lord." (Commentary on Tit. 1.7, quoted. in “Religions of authority and the religion of the spirit," pp. 77,78. 1904, by AUGUSTE SABATIER. A similar translated version of this is provided by "Catholic World," Volume 32, by the Paulist Fathers, 1881, pp. 73,74).
But while Rome invoke the "unanimous consent of the "fathers," she picks and chooses among what is actually not unanimous consent.
And consistent with the inconsistency such with Scripture, what Jerome (among others) taught about marriage is certainly not what the NT church taught:
On First Corinthians 7 Jerome presents this false dilemma
"It is not disparaging wedlock to prefer virginity. No one can make a comparison between two things if one is good and the other evil ." (''Letter'' 22). "It is good, he says, for a man not to touch a woman. If it is good not to touch a woman, it is bad to touch one: for there is no opposite to goodness but badness. But if it be bad and the evil is pardoned, the reason for the concession is to prevent worse evil."
But Paul is not teaching that marriage is evil yet is allowed and pardoned prevent worse evil, but both are good, yet celibacy is better - for those so gifted - as regards pursuit of personal holiness, especially considering the imminent judgment to come.
Then we have this reasoning:
"If we are to pray always, it follows that we must never be in the bondage of wedlock, for as often as I render my wife her due, I cannot pray. Which excludes sexual relations as being an expression of Christian love, and if anything that prevents concentration on prayer is to be rejected then a whole list of things Scripture sanctions can be added.
And then we have this wresting of Scripture to teach marriage being unclean:
This too we must observe, at least if we would faithfully follow the Hebrew, that while Scripture on the first, third, fourth, fifth, and sixth days relates that, having finished the works of each, “God saw that it was good,” on the second day it omitted this altogether, leaving us to understand that two is not a good number because it destroys unity, and prefigures the marriage compact. Hence it was that all the animals which Noah took into the ark by pairs were unclean. Odd numbers denote cleanness . St. Jerome, Against Jovinianus Book 1 http://www.ccel.org/ccel/schaff/npnf206.vi.vi.I.html
So much for 2 x 2 evangelism, while "if we would faithfully follow the Hebrew" "God saw every thing that he had made, and, behold, it was very good. And the evening and the morning were the sixth day." (Genesis 1:31)