Photo Library Issues

Don't know why, but my Photos library (~500GB) on my Mac got messed up. At first Photos wouldn't rebuild it, so I changed the library's name and then used TimeMachine to recover the most recent backed up version (two days old). When I attempted to open that library, Photo had to rebuild it (took 10 hours). The rebuilt version had all the photos, but lost connection to all my shared albums - shared albums that still show up on my iPad and iPhone as well as my wife's iPad and iPhone.

I then tried once more to recover my original library; this time it was able to (again taking around 10 hours). What is interesting is that if I changed one of these two libraries, the other one updated. I.e., if I added photos to one the other also had the new photos. At this point I deleted the previously broken library.

Then I created a new shared library on my Mac and it shows up on my iPad - along with my old shared albums - and my wife was able to join it.

How to I get my Mac's Photos album to reconnect with the old shared albums?

Why did Photos (1) have to rebuild my Photo's library when I recovered it from TimeMachine, (2) lose the shared albums in the first place, and (3) show "0 Photos Scanned" when I had a large number of people in my library? Isn't TimeMachine supposed to prevent these sorts of issues?

And finally, the new library is really slow to use. I tried to modify the location of some new photos I imported and it can take many minutes for the location information to load and then allow me to modify it. Is it because the library has to rescan all 49K photos?

This is really poor software.