Need advice for testing of hypotheses related to the self-citation method

[Jorge_Stolfi]

And we don't know if it's a hoax or not, so it's ? in 1.

Agreed!

1) There are reasons to make a hoax, and there are historical examples of hoaxes/forgeries

Indeed. And, just to mention in passing, there are examples of dealers "enhancing" a possibly genuine artifact with bogus signatures or stamps in order to increase its market value.  For instance:

• "You are not allowed to view links. Register or Login to view. inscribed by Auden to Clare Boothe Luce, one titled Pre-Columbian Historical Treasures, the other an American edition of Letters from Iceland. Both signatures are forgeries"
  
• "You are not allowed to view links. Register or Login to view. “A Modest and True Account of the Proceedings Against Mr. Abraham Anselm,” published in London in 1694, was reported missing from the Library, and four years later, now enhanced by some legal notes supposedly written and signed by Lincoln on one of its end papers, possibly to make it appear that it had once been part of the President’s library, it was picked up in a bookstall on 125th Street by a collector named Otto A. Hicks, who took it to Bergquist for verification of the handwriting. Bergquist was saddened to see that the end paper was clearly another bit of Coseyana.)"
  

While the MS is long, it's possible to find a scenario which would call for a long forgery (say, VMS should have represented the original of a foreign manuscript of roughly known size)

Yes, there have been such cases too.  The You are not allowed to view links. Register or Login to view. is a recent one.  More relevant to the VMS are perhaps the "Enochian language" books given by "Angels" to John Dee, through a crystal ball and "recorded", by his in-house scammer Edward Kelley.

The length of the VMS by itself is not a problem.  The problem is that the forger wrote 250+ pages of a complex invented language and bizarre illustrations, without adding a single element that would have made the book more attractive to the intended victim -- like some recognizable alchemical symbols, pictures of sick people being cured, intriguing weapons sketches, etc.  And he also choose to use all the vellum that he had, instead of discarding or trimming parts that were obviously bad (like f68r4, f72r4, f102r3, f112r...)

All the best, --jorge