The Modern Forgery Hypothesis

[ReneZ]

That would be flawed reasoning. In both of your scenarios, you have declared the letter genuine a priori.

I did not assume anything about the letter, only about the date range of the watermarks. 
Still, even if I did, it would be the right starting point.

99.999% of all historical documents are genuine. (Yes, I invented that number but this is the idea. The truth is probably higher).
This means that this is the default, and the alternative proposals needs strong evidence.

The watermark says nothing about whether the letter is genuine or not. It does not allow to decide either way.
It could have been different, in case it appeared to be much later, but this did not turn out to be the case.

The same is with the C-14 dating: it could have shown very damning evidence, but it did not.
Also the chemical tests of McCrone: it could have shown very damning evidence, but it did not.
(The latter is one of the more common ways how modern fakes are detected).

On a side note, I find the suggestion that the Marci letter should be a fake far more challenging than that the Voynich MS should be a fake.