but it's still work someone has to do, and there's many requested features - as well as maintaining the current feature set.

I mean, sure. It's a community project too, right, so turnaround is even longer probably. I know it's not going to happen at the flip of a switch, I'm a software engineer by trade. I'm just saying that Plex handles this while Jellyfin does not, so I'm going to use Plex until this works in Jellyfin.

I was looking into the Merge Versions plugin, but I'm hesitant to use it if it means that it'll go into my library and rename all my files. I don't want that. But I couldn't find a description of how the plugin works, so I'm holding off for now.

Sadly there's an internet connection required to login to Plex, which is a no-go for me.

I don't think this is necessarily true? You can manually enter the server address without logging via plex.tv, I believe.