Why is the MS not a copy of older, non European sources?

[Koen G]

I would say the most important evidence toward that is the last line of You are not allowed to view links. Register or Login to view. marginalia which mixes Voynichese with some Latin European (presumably German) stuff. This suggests that the marginalia author (a 15th century or later guy) possessed knowledge of Voynichese and made use of that.

I see the script as a late addition, which may have been devised by the people copying - or in that case, transcribing - older sources into MS Beinecke 408. So these marginalia point to that conclusion indeed. But if we assume a recent teanscription effort, observations on the script alone are just that. They allow us to date the script, about which I agree with your impressions.

JKP - sorry, I was speaking in very general terms. Your stance is relatively open and mature, and you clearly have a good understanding of how medieval manuscripts often used a number of varying sources.