The Modern Forgery Hypothesis

[Mauro]

If the hypothetical book described by Baresch were to be found today, then all of the provenance that we have for the VMS between ~1438 and 1912 simply collapses.

No no, sorry. If that book were to be found just a rather weak (*) piece of evidence for the VMS being written in XVth century would collapse.  And, by the way, the Baresch letter does not estabilish provenance, it just estabilishes a terminus ante quem.

The bulk of the evidence would remain intact: C14, which does not prove VMS was written in the XVth century, but which makes it highly probable, the unusual steps the faker must have taken (yet again possible, but highly improbable), and the overall physical status of the manuscript (ditto for probabilities), + miscellaneous facts (ditto&ditto). The modern forgery hypothesis would need a lot more evidence, besides finding the yet-to-be-found Baresch manuscript, before becoming viable.

(*) weak, because we cannot prove Baresch was actually talking about the VMS