Speculative fraud hypothesis

[ReneZ]

I do not find this a bad idea at all, and in fact it has also been discussed before.

This is close to a hypothesis from a reputable expert on ancient herbals: Sergio Toresella, described in this publication: Toresella, Sergio: Gli erbari degli alchmisti, in: L. Saginati, Arte farmaceutica e piante medicinali; erbari, vasi, strumenti e testi dalle raccolte liguri, 1995.

A few comments on some of your pros and cons:

Allows for collaboration with common purpose by multiple people, explaining multiple scribes

Very subjective, but this is not so obvious to me. Possibly if he paid a group of scribes in order to have it finished quickly.

The parchment appears to have been acquired and prepared similarly, not congruent with being built up piece-by-piece (though not impossible)

We do not know if the parchment was acquired all at once or piece by piece. It may have been acquired over a 20-year period. The dating is simply not accurate enough.
I never heard any opinion on whether it has been prepared similarly.