PRIMER NO. 1.
WHY IS THE CHESHIRE PAPER UNSCIENTIFIC?
Note: The paper entitled "The Language and Writing System of MS408 (Voynich) Explained", written by Gerard Cheshire, published in the Journal of Romance Studies, and located at https://www.tandfonline.com/doi/full/10.1080/02639904.2019.1599566 is herein called the "Cheshire paper."
No doubt you have been drawn here by a mention of the Cheshire paper. We got into this because of one too.

But after serious examination, we found its argument to be completely untenable.

The paramount reason it should not relied upon is its basing entirely on conjecture. And if every other reason was proven untrue, this reason would still stand. It is true that all research has to come from somewhere, and all original ideas have no citation. But no description of how Cheshire came about his conclusion is set out anywhere in it. It is a system of unfounded ideas, and, as we shall see, simple assertion is insufficient to prove such bold hypotheses.

First, is that there is no Proto-Romance language distinct from Latin.

The breadth of the Roman Empire meant that practically every language spoken within its Western parts was abandoned by its speakers for Latin. The diversity of languages that had prevailed before the Romans conquered today's Romance regions wholly ceased to be replaced by Latin. The common population made changes in their Latin, creating a colloquial variant called Vulgar Latin, with different grammar and pronunciation, that was not reflected in formal written Latin. As the Western Empire fell to barbarian invasion, each of the invading tribes sought to emulate Roman customs and language. Their corruptions of the pronunciation and grammar of Latin form the basis of today's Romance languages. A new diversity of languages sprouted on the corpse of the terminator of the old diversity of languages. (Davis)

Second, is that there is no reason Ischia would adopt its own writing system when there were others already.

Ischia is located but a few miles off Naples, a bustling city. Cheshire claims this was written in 1444, shortly after the Aragonese conquest of Naples, during the Renaissance. All over Europe people were writing how they talked, as well as in Latin. Even in Italy there was Dante's Divine Comedy, which could not have been unknown to people of means anywhere in Italy, which showed that vernacular languages could be written well with the Latin alphabet.

Different countries in Medieval Europe did have different writing habits. However Cheshire's assertion of there being one for Ischia does not hold water. A large nation can afford to develop its own peculiar writing habits, as the larger a nation is, the more trade will take place from place to place domestically, and the more writing will be done relating to internal administration and the less proportionally relating to foreign diplomacy. The literate people on Ischia would be almost entirely reliant on trade and intercourse with foreign countries, as their island was so small.

Moreover, unlike islands like Jersey and Malta on which separate languages did develop due to political separation, there was no political separation on Ischia. Historically it changed hands roughly lockstep with Naples. Ischia was never an independent polity, and always a dependency of some other state. Geographical proximity and political dependence, make economic reliance, make linguistic similarity and orthographic identity.

Third, is that Cheshire does not follow recognized practices of comparative linguistics in making his analysis.

We have alluded to it in the first point, but this bears further discussion. Let us illustrate the pitfalls of not following them with an example. Every Romance language has the word "cafe". Did the Ancient Romans have cafes? Obviously not. Instead, we must recognize that words descend from ancient languages into modern languages by certain predictable patterns. A word which appears in more than one daughter language, in a form which is conformable to the rules by which words descend into each, can be said to appear in its mother language, and not a mere cognate between the two languages. Cheshire has provided no rules whatsoever by which words in his Proto-Romance descend to such varied descendants as Portuguese, Romanian, and French.

We may yet assume that Ischia managed to conjure the need for their own writing system out of thin air. We may yet suspend our disbelief of the ability for words of Ischia to travel to Portugal and Romania, when they couldn't even get to Naples. We may yet ignore the history of how Romance languages came about, known to every high school student. But we would not be reading a scientific paper. We would be reading a story. The proper course of rational thinkers is cut it all away with Occam's razor.
REFERENCES/FURTHER READING:
Gerard Cheshire, Vulgar Latin, and the siren call of the polyglot… by Nick Pelling. Available at http://ciphermysteries.com/2017/11/10/gerard-cheshire-vulgar-latin-siren-call-polyglot .
Cheshire reCAST. Available at https://voynichportal.com/2019/05/07/cheshire-recast/
No, someone hasn’t cracked the code of the mysterious Voynich manuscript by Jennifer Ouellette, with a pertinent quote from Lisa Fagin Davis. Available at https://arstechnica.com/science/2019/05/no-someone-hasnt-cracked-the-code-of-the-mysterious-voynich-manuscript/