Interchangeable ch/sh and k/t characters

[Ruby Novacna]

...we know that Voynichese glyphs do not behave like vowels and/or consonants in French, English, German, Latin, Greek, Italian etc.
... the differences with European natural languages are so huge that I think you can get the idea in any case.

Marco, I'm not trying to convince you; I have no counter-argument to offer, especially when you're comparing the text of a single manuscript with a language like French.

That said, I agree that the EVA glyphs t and k are sometimes difficult to distinguish, so the scribe could have confused them if he didn't truly master the language.
Personally, even in dictionaries, I struggle to distinguish between Coptic n and p, Greek omega and pi, sigma and stigma, and I rarely recognize the subscript iota, etc. 
If you ask me to copy a manuscript, I'm likely to make many mistakes.