... 5) there is zippo reason to endorse Jerome authorship of the VP. None.
The factual error is your claim. There are many solid reasons to "endorse" Jerome's authorship of the Vulgate Prologue to the Canonical Epistles. Here are three biggies.
1) it is a first person writing, addressed to Eustochium, a frequent correspondent.
2) the theory of Jerome's non-authorship
a) developed very late, and was largely based on the animus to the authenticity to the heavenly witnesses verse
b) was originally based on the supposed late date of the Prologue manuscripts, which fell apart by the Fuldensis discovery
3) There is no good forgery author(s) proposed, all such speculation has been extremely weak.