My Solution – Abbreviated Middle High German

[Rafal]

Regardless of whether my translations are correct, do you think it is possible that the author could have written the manuscript in this way?
 
In concrete terms:
• with abbreviated word forms,
• by renouncing part of the grammar,
• with individual abbreviations,
• as a kind of personal notation or memorization style,
• with the aim of making the content difficult for third parties to understand, but still being able to read fluently.

I think a better question would be if it is probable than if it is possible.
And I would say it is possible but rather improbable 

Let me make a comparison from modern times, people interested in cipher riddles could know it:
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I would say it fits your description. Ricky was a half-illiterate man who developed his personal way of writing down things, shorthened, ungrammatical and understood only for him. Yet after all it seems to make some sense, he was probably writing about his meetings with some people, see:
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But I would say there is a difference between notes of some petty swinder and long, illustrated manuscript written in a tidy way. People often make dirty notes but they rather don't write big works in dirty notes.

There is also a serious problem with a theory of some illiterate man, madman or autistic kid. Voynich Manuscripts seems to be a collective work with several scribes involved. And they probably had access to many other manuscripts which they used as inspiration for their illustrations. VM may be poor quality but making it still involved time and money.

We simply don't know any cases where someone would take such coordinated effort and produce thing of such scope which would sound like babbling of a person with brain damage.