Why and how the text could be Bavarian

[Jorge_Stolfi]

I don‘t know of any book or script that ever came completely without any doubled consonants.

But all the German spelling systems you know are variations of the same general spelling system, derived from the Latin script and using mostly Latin letters.  You don't know any example of someone inventing an original alphabet and original spelling system for German. 

Since Old English, most texts in English used variations of the same spelling "system" (if we can call it that), using mostly Latin letters, that ultimately evolved from Latin spelling.  But there were a few radical spelling reform proposals that would have used a new alphabet and completely different spelling.  The least unsuccessful of them was the You are not allowed to view links. Register or Login to view..  Most if not all of the doubled consonants of standard English spelling become single consonant signs in Shavian:

"? ???? ??? ? ??? ??? ? ????? ?????? ?????."
"the batter hit the ball with a sudden clapping racket"

[EDIT: This wonderful forum software turned all the Shavian letters to "?", even though my browser was showing them fine in the edit window. Sorry. Ask the Internet to render that sentence in Shavian.]

That's because, in the standard English spelling, the double consonants are generally used to change the sound of the preceding vowel (like German, as you say).

To be clear: without clear indicators there is not any chance to separate „Rate“ from „Ratte“. Some „just phonetic“ script can never exist due to this structural problem.

If the sounds and meanings are different, a phonetic script should have a way to distinguish them.  Logically it should be a mark on the vowel, rather than on the consonant.  The Internet says that the IPA notation for those two words is "Rate" = ['ʁa:tə] and "Ratte" = ['ʁatə], where the «'» indicates that the next syllable is the stressed one, and the «:» indicates that the preceding vowel is long.  A more conventional way could be "Rate" -> "Rāte", "Ratte" -> "Rate".  Or "Rate" -> "Rate", "Ratte" -> "Rǎte".

 I know [stenography] as a complex system of abbreviations for syllables, words, and parts or complete (often used) sentences. Stenography exceeds the few symbols of Voynichese by numbers

That is "advanced" stenography.  It requires memorizing all those abbreviations.  Basic stenography is a usually just a phonetic alphabet with simple letter shapes, like Shavian.  Usually "cursive" so that a whole word can be written in a single stroke, without lifting the pen from the paper.  (In fact, that is actually the point of ordinary cursive script, too: to be faster to write than "print" letters.)  

For a book that was intended to be read by others (as I believe is the case of the VMS), it would make sense to stick to such a "basic stenography": a phonetic alphabet with simple letter shapes, with few if any abbreviations.  Especially if the Scribe had to be trained to copy the script.

All the best, --stolfi