Hey lazorbeam, I am happy to hear you like my app
Regarding your questions:
1) Due to the way the list is maintained internally you can not sort by download state, but there is a filter option for downloaded books in the pro version.
2) The only long-pressable fab is the layout change button in the main list, which will give you the options dialog for that list.
3) The "Update" download options work just like the regular ones with the difference that they force a download even if a local copy exists and seems to be up to date. This is to make sure you can always easily get to the newest file version if you update/polish/otherwise change the file as the calibre library does not include change timestamps for the files themselves, so I can oly use the filesize to automatically overwrite the file with a changed version. So you will probably only need that button very rarely, if ever.
The book metadata is part of the calibre library, so the only way to apply those updates is to update the entire library in the home screen.