First issue: the multiple libraries in the folder chooser. This happens because of a bug I ran into myself 2 days ago. The extra libraries have been deleted in Drive, probably because you did what I did: deleted the library then copied a new one with the same name. Unfortunately with the current CC version there is no way to determine which library is really there. Try all three, although I am not convinced that it will work. Another thing with perhaps a better chance of working is to permanently delete the other copies from Drive's Trash folder.
The next CC release fixes this bug. I would make a release now, except that I just switched development environments because Google decided to stop supporting the one I was using. I am not sure if I broke something. I was planning to run through a quick beta cycle before making the release generally available. I can send you an intermediate version that you would need to manually install. It is an "official" version so it won't require uninstall or reinstall. If you are willing then send me a PM with the email address where I should send the APK. Be sure to tell me which version (demo or paid) you are running so I can send you the correct APK. [EDIT] If you have been using CC for years then it is the paid version, given that the demo version hasn't existed for years.
Back to Drive: CC is apparently finding a metadata.db file. What we don't know is which one. The way Drive versioning and parenting works it could easily be the correct db in the incorrect library.
Second issue: assuming that the above doesn't fix it then I will need a CC debug log. When you try to download a book the path in the calibre library is logged. In fact, if you send the log to yourself then you can verify that the path CC finds in the metadata.db file really exists in the library. If there is nothing obvious then forward the log to me, cc_debug_logs [AT] charles [DOT] haleys [DOT] org. I might see something there that helps find what is going on.
Another possibility: CC will fail like that if your books are stored on an SD card and for some reason CC cannot write the book. Can you download a book with the content server with the same CC installation? If so then the cloud connection should have no problem.
As for uninstall and reinstall: that is a last resort that probably won't fix anything. You will lose your CC database, requiring you to resend all the books.