1) No, unless you mean jump from JoiPlay 1.20.410 (and older) to newer runtime based JoiPlay 1.20.500 (and newer), then old plugins (e.g. Ren’Py Plugin 8.1.3_7.6.3_001 and older) can be safely uninstalled after installation of new plugins (new Ren’Py Pluigns with specific Ren’Py versions).
2) A successor to the “Compatibility List” to check users’ feedback on the playability of games
3) “Game Catalog” ig? Generally, it’s better to search by engines that are supported, e.g. you can do such searches in VNDB, SteamDB, Itch.io, etc.
Here’s some places for downloading games