You are absolutely correct. Global would not be much help when alot of people have the same movies with different quality(720p/4K/etc) in separate libraries.
Anywho… For giggles(and to gain more knowledge), I just tested(ThePlexDance) both a movie and an entire TV series and they both restored their viewstate. My movie showed I had already played an hour and 10 minutes with 24 minutes left. And my tv series shows season 1 has been watched and season 2 is queued OnDeck to be played.
So it safe to assume you will be fine and your play/viewstate will infact restore if you plex dance your content.
One caveat though
You MUST use the same matching agent you used previous. Meaning, if your movies were matched with agent “TheMovieDb” then continue to use that one. Same goes for tv shows. If “TheTVDb” was used, then continue to use that.
