Which other manuscript contain the most Voynichese glyphs?

[-JKP-]

So... might I paraphrase that close familiarity with "is"-abbreviations could have created the mindset required for a scribe to play around with (meaningful?) loops added in a systematic way, thus giving rise to the gallows? As well ad some of the more fanciful/decorative loopy shapes added to some initials maybe.

Yes. Exactly, although I suspect a few of the loopy shapes are embellishments (which were also relatively common, especially in legal documents).