The nymphs are possibly inspired from Dante manuscripts

[eggyk]

Weren't (most) MSs in the middle ages simple copies of existing works, instead of a mish-mash of tens of different sources?

I would somehow agree.

Medieval books often feel very repetitive. For example each Book of Hours is unique but at the same time they boringly copy the same motifs all the time.

And Voynich Manuscript indeed seems to be a true mish-mash of many, very different sources. A kind of medieval collage. It's quite unique in that aspect.

Books of hours were indeed all basically the same, but they were used for a specific type of purpose so wern't really open to being changed. But go over to germany, and a huge amount of manuscripts contained a compilation of various works and texts, basically depending on which works the person wanted. So much so that a title like "Collection of various astrological works" is normal to see. It's not always imagery being copied, sure, but it was certainly normal to copy and bind together seperate works. 

HSCensus is great for showing this as some works are tagged, like "You are not allowed to view links. Register or Login to view." or "You are not allowed to view links. Register or Login to view.'". They appear in various different compilations.