I can only really speak to your first point. When imported my existing library, I did it using Sonarr/Radarr as applicable. They have a manual import method, here’s a description of Sonarr’s.

Unfortunately that’ll probably work best if they’re formatted in a way Sonarr can readily recognize, something like /Season ##/S##E## - .ext. It may take a little work to get there, I found a program called mmv which helps out a lot. It allows you to move files that match a pattern, capture parts of pattern, and use that captured part to name the output file. That allowed me for format entire seasons at a time, but that method does rely on most files having similar names to begin with.

Since Jellyfin v10.6.0, it’s had a feature called SyncPlay allowing multiple users to watch the same thing at the same time (coordinates pauses, fast forwarding/rewinding, and all that between clients). I’ve used it and it worked like a charm, although I did find that not all clients support it.