The Modern Forgery Hypothesis

[ReneZ]

As you and Jan asked, why did neither Baresh nor Marci mention Jacobus, if his signature was readable on f1r?  Baresh knew Jacobus, at least by fame, and would have recognized his name even if it was only barely visible...

First of all, this is guessing and double-guessing. It is better to go by the evidence, and there is quite a bit.

There is written evidence that Garland suggested the Bolton book to Voynich in 1921. There is no evidence that Voynich knew of it before. It is clear from this correspondence that Voynich used this book in 1921.
 
There is also written evidence that Voynich was not the least interested in later ownership of the MS for the first nine or so years that he owned it. He repeatedly stated that the Rudolf in the letter was Rudolf I, a contemporary of Roger Bacon, and even ticked off people suggesting that it was Rudolf II.
He only cared about Roger Bacon.

The only person of any relevance for the Voynich MS in Bolton's book is Jacobus Horcicky de Tepenec.
His name in the MS is written like this: Jacobus a Tepenecz.
In Bolton's book he is only named as Horcicky or Sinapius. 
The name "Tepenec" does not appear in the book. (The PDF is online and searchable).
Voynich could not have obtained this name from Bolton, and Horcicky is only one of many many dozens of people who could have been of interest.

This is also why Voynich was so exuberant about Garland's discovery that Horcicky (in Bolton) and Tepenec (in the Voynich MS) are the same person.

One can build an entire structure of hypotheses based on suppositions, but it is not impressive if these also contradict lots of evidence, that would then require one to state that these are all lies.