The Book Switch Theory

[ReneZ]

Also notice that some circularity has snuck into the exercise

This is a good observation. A clear example of that is the suggestion that Voynich cannot be trusted in any respect. 
*If* he added a faked signature in the Voynich MS (or significantly worse, as per the other theory), then by necessity he is a 'crook', a faker. 

This is a consequence of the theory. It is not a given.
It can be considered a hypothesis that he was dishonest to that level, but hypotheses cannot be used as evidence.

Yes, he exaggerated the value of his merchandise. Also, he did not disclose where he obtained it. This is probably common to most second hand book (and later car) salesmen. Just repeating what I have been told here :-)

Voynich faked 'something' because he was dishonest, and he must have been dishonest because we think he faked something, is a clear example of circular logic.

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On a side note, many a Voynich theory exhibits some of the same problems as outlined above.
The points in favour of the theory are exaggerated. The points against it are simply not addressed.
The talks and papers are not consisting of weighing evidence pro and con and deriving conclusions, but appear like a sales talk for a bad product.

Just to avoid misunderstanding, with this I am not specifically thinking of the book switch theory, but numerous others.