You make a very interesting point. The OP refers to writings of early church fathers that are not found in the bible to make the case of Mary’s perpetual virginity, and goes on to describe purely Catholic experiences; visions and apparitions. Holy Tradition.
1. First of all, we know St. Jerome lived and we know he wrote on the topic of the Perpetual Virginity of the Blessed Mother. So, we know that the early Church believed in this.
2. Actually, I tried not to use “purely Catholic experiences” for this post. Most Protestants do not totally discount the writings of the ECF and to my knowledge, Martin Luther believed in the Perpetual Virginity of the Blessed Mother as well.
3. Because there is much to prove the apparitions of the Blessed Mother, I really don’t feel these are “purely Catholic.” Which is why I used the example of Our Lady of Guadalupe which has received some popular attention, even on television. Just google apparitions of Mary, in Zeitoun, Egypt. There are pictures! She was seen by millions - Jews, Muslims, Orthodox, Catholics, Atheists…Really, this stuff is out there for all to see. Not just Catholics. (Unless we choose not to then we can stick our head in the sand and call it “purely Catholic”).
That’s not a problem at all—
if one is Catholic. However, Protestants generally had major problems with Holy Tradition; they accepted some of it, rejected a lot, if not most, of it…and refused to use any of it as prooftext material for doctrine. In fact, for a Catholic to attempt to prove the truth of a doctrine to a non-Catholic by using Holy Tradition is very much like a Mormon attempting to prove the truth of a belief by quoting from the Book of Mormon.
**No, because there is very little that the Mormons do today that is in the BoM. **
Both extra biblical sources of doctrine are great for proving WHAT beliefs are, but neither are any good for proving to non-believers that those beliefs are ‘true.’ First…get everyone to agree that those extra-biblical sources are appropriate sources, and
then use them.
I disagree. Look up the pictures of the apparitions in Egypt and read the stories of the millions who saw her. Read about the image of Our Lady of Guadalupe, then tell me it is only for Catholics!
Catholics have a bigger row to hoe here with Protestants than they do with Mormons. At least we accept the idea that doctrine and beliefs CAN be found in extra-biblical sources.

The
idea of 'Holy Tradition" doesn’t bug us at all. It’s just that we believe that much if it is, well…apostate.
What Catholics must do, in speaking to Protestants about the virginity of Mary,(that is, in trying to prove it) is to use only those sources that all participants in the discussion accept. Using quotes from Holy Tradition is fine–to show why Catholics believe. It’s not so good for convincing those who dismiss Holy Tradition that they must believe, too.