Don
I don't have much time, about to travel abroad tomorrow. Just some quick remarks though.
The text: I agree that it looks very Latin. This does not say much about origin, though, since it might be a transcription. In fact, if the script is constructed, it is almost by definition a transcription. I have analyzed a number of labels from the "small plants" section and I take them to contain the "local" names of tropical plants, aimed at Greek speakers. One of the possibilities is that it was originally written in Greek, then much later transcribed to a more Latin set of glyphs. Just to say: I agree that the glyphs do look Latin and to some extent contemporary to the physical manuscript. That does not mean, of course, that they are
original.
The imagery: I have focused my efforts mostly on the small plants section and quire 13. In those sections, I see not much that points towards 15thC "Latin" influence, and, as you know, much that points South-East rather than North-West.
About the other sections, the ones you mention, I mostly agree with Diane's analyses. The nine rosettes foldout is special case and has undergone different influences than the rest of the manuscript. I must admit that I don't understand the first thing about it
Now the "zodiac which may not be a zodiac", that's something else. It has clearly undergone the most recent edits. In fact, over at Bax's site, I have myself participated in finding parallels for the Gemini image, and those are found in the Carolingian and later traditions. Also, it would be weird to claim that the crossbowman is Hellenistic Egyptian
What I find the most important thing to bring across, is that, if we assume much older sources, the manuscript is kind of an "anthology", gathering different sources that have known a different history, transmission and cultural influences. Many of such older manuscripts are anthologies much more than the work of an "author".
So for me it is perfectly fine to say that yes, one section shows 15thC medieval influences, while another doesn't, and yet another has been gained from a totally different place.
A consequence of this view is that the text
must be either a late addition, or a late "uniformization". Because that is one of the things that unify the manuscript.
So in short, I actually agree with most of your points, which doesn't mean that there must have been a 15thC European "author".