Edit-Distance computational attack idea

[Sam G]

That the VMS is not encrypted is basically proven by the low second-order entropy of the text, since virtually all ciphers increase entropy.  The main exception, verbose ciphering, is ruled out by the lack of repeated strings (no long words and no repeated sequences of short words).

Is the fact that all ciphers increase entropy mathematically proven? I think it is not. So there are no foundations to state that. One may state that "all ciphers known to this person increase entropy", but that would not disprove the hypothesis that VMS is enciphered since it well may be enciphered by a cipher unknown (indeed, if it were enciphered with a known cipher, it would have probably been deciphered long ago).

So it is by no means "proven" that VMS is not a cipher.

Whether it can be mathematically proven or not I'm not sure, but there is certainly no such cipher known to modern cryptography.  So you're left arguing that someone in the early 15th century invented some entropy-lowering cipher that was never subsequently rediscovered (not even with the VMS to serve as an example) and indeed that appears theoretically impossible today.

For the verbose ciphering, is it ruled out? What if it is supplemented by shuffling?

Simple verbose ciphering is surely ruled out, assuming a plaintext in a European language.  Shuffling will increase the entropy at least somewhat, and at the very least there is no known procedure that does this.  To a large extent verbose ciphering is like steganography, which is basically impossible to disprove for any text, though there are strong arguments against it in the case of the VMS.