Emby also seems to be able to automatically merge different versions of shows/movies.

I've asked someone and MergeVersions does not change the file/directory structure/naming.

Though the same person also mentioned issues with duplicates even after removing the second library with UHD.

In general plugins like IntroSkipper and PlaybackReporting work flawlessly but I'll stay away from MergeVersions for now and handle movies with mergerfs for the moment.

You're right, it's possible to login without internet if local network access without login is enabled in settings beforehand.