The Latin Vulgate Prologue would likely be subject to more textual corruption and change than the Latin Vulgate text was (especially it was supposedly transmitted separately),
This does not make any sense. The Latin Vulgate text was subject to the wide variation in the Old Latin manuscripts, 2nd century on, even leading to hybrid manuscripts. The Prologue was a singular text, and the time period was much shorter, so its variation would be far less.
Someone else could have written it to try to advocate the later form or version of the Latin Vulgate text that he favored.
What is the "later form"?
Anyway, there have been theories of other authors, like Peregrinus, but they have been shot down.