Was the paint added later? And is the artist also the scribe?

[Bernd]

I think the observation that the same pen might have been used for text and drawing is worth investigating And a page-by page assessment would be very valuable. Which pages have a match, which don't?

Still I think it's not that easy. In the beginning I thought I had found a strong correlation between drawing stye and scribal hands but there are also extremely striking similarities across hands that are very difficult to explain with different untrained artists sharing the exact same style.
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For example the strong similarities between the small root faces on f89r1 (Hand 1) and the small astro faces on f67v2 (Hand 4). 
Same for the root face in f101v2 that resembles zodiac faces like the crossbowman (Hand 4)
 
And the root faces in You are not allowed to view links. Register or Login to view. (Hand2) closely match the large astro faces (Hand 4).
I see no difference in ink suggesting those were added later. As Lisa pointed out, while there are clear differences, all human figures and faces likely were drawn by the same person. Same goes for stars by hands 2,3,4.

I still don't know what to make of this but my personal opinion is that a single author for all text and imagery is the most parsimonious hypothesis to reconcile all this oddness.