@Staudey you don't get enough thanks for staying on top of this, so thank you. I've not seen your comprehensive edited update lately.
For me
#2 still a miserable experience but I'm in all the recommended themes. Is simply uninstalling/installing Nautilus an option here, or irrelevant towards a font fix?
#12 and #13 fixed for me, thanks
#17 not fixed for me. drop a file and poof you're in the new folder
I have a #21 that's pretty common now: launch nautilus but no gui/rendering/window. the dot is above it in icon bar so it's alive, and budgie seems to think there's a window there since you can put it in front of opens apps and behind open apps but it can't be seen. No frame no outline no file manager. it's active in system monitor so I know it's running.
All your advice re: themes has been followed by me (materia/korla).