[sciurius]

It seems to me that after the initial tests it becomes clear there are some aspects that need rethinking. That's fine, that's what these tests are for.
Identification of songs across devices is crucial but the current approach is unfortunately not sufficiently fullproof.
If you agree that synching between arbitrary song libraries has little or no use, assigning uuids to the library and the songs may solve most, if not all, problems.

I'm a bit surprised to hear that a full replace using synch would require a lot of additional work, since that logic is already part of the library backup/restore functionality.