Lidarr is centered around full albums unless a song was released as a single, specifically it uses release-group on musicbrainz.
I run both jellyfin and Plex, and for the music app I think plexamp > finamp, but both work to sync between their respective instance. I haven’t tried anything else because I already had Plex pass for other things.
If the torrents are still seeding when the import in arrs happens, it would copy the file(s) to your media folder so you’d have two copies now. If the torrent is done seeding and, I think if you have completed download handling enabled, it’ll do a move instead so you’d end up with only one copy.
Ideally though you want a hardlink compatible setup but from what you said you do not.
Jellyfin just monitors your media folder which is managed by the arrs.