Interchangeable ch/sh and k/t characters

[Jorge_Stolfi]

my example uses a very diverse panel of texts.

Those texts are "very diverse" only from the literary point of view; they are all very similar from the linguistic one.  They are all running narrative/discursive prose, with long complicated sentences, that try to be varied, elegant, and informative, etc..

For these reasons, they all have a broadly varied vocabulary, but mostly of "core" (not subject-specific) Italian words, and make heavy use of mostly the same function words -- articles, prepositions and conjunctions, pronouns, copulas, auxiliary verbs, etc.  These similarities are not significantly affected by the subject matter, epoch, authorship, or even poetry x prose x theater.

And, again, the frequency distributions D1 and D2 of letters and bigrams are largely determined by the presence or absence of those letters and bigrams in the most common words in the text.  And the character entropies H0 and H1 are just projections of those distributions. 

And, as you note, the Italian (Tuscan) language has changed surprisingly little from the 1300s, and the standard Italian orthography has hardly changed at all.  Thus it is no surprise that D1 and D2 of all those texts cluster tightly together in those character statistics.

That would not necessarily be the case of texts that are not "literary". Like ship logs, accounting books, parish civil registers, catalogs of products, kings,  stars, saits, and churches,  ... or succinct herbals.  Even if they are definitely in Italian.

It also would not be the case if the language went through a significant spelling reform.  Portuguese had one in the 1940s.  Here are three versions of the same novel (Dom Casmurro by Machado de Assis) which are the exactly the same (same words in the same order, thus the same slightly outdated syntax), only in different spellings:
You are not allowed to view links. Register or Login to view. original edition, old spelling.
You are not allowed to view links. Register or Login to view. a recent edition in (almost) current spelling.
You are not allowed to view links. Register or Login to view. a more phonetic transcription of the modern Brazilian reading.
(Beware that these files are in ISO-Latin-1, not Unicode; and this detail matters, because they have accented letters that are incompatible with UTF-8 encoding.)

For comparison, You are not allowed to view links. Register or Login to view. is a Spanish translation (literary, not word-for-word) of that novel.

In my opinion, Portuguese orthography is the second-worst among the languages I know something about.  Worse than the French one, and second only to English.  The reform of the 1940s could have made it at least as phonetic as the Spanish one; but it was half-hearted, and kept many of the worst features of the old script -- like the random and unnecessary use of "ç" for soft "s", the silent word-initial "h", and the four different pronunciations of "x".   But it did remove most doubled consonants, many silent consonants like the "c" in "acto" and "p" in "optimo",  replaced "ph" by "f", and replaced some diphtongs, like "eo" by "eu".  It also added or removed many diacritics.  

So the letter and bigram frequencies of the "1899" and "1999" versions above should be noticeably different.  Maybe not enough to make them "different languages" by your criteria?

The "2099" version is my idea of what a real spelling reform of Portuguese (rather, Brazilian) should have been.  It is as phonetic as I could make it while staying within the Roman alphabet and ISO-Latin accented vowels. The letter and bigram statistics should be quite different from the other two.   I expect that, by your criteria, the differences between "1999" and "2099" will be greater than those between the former and the Spanish translation above.

I still owe you an example of a text in Italian that would not be free prose like those in your list.   Working on that...

All the best, --stolfi