I’m not familiar with Kid3, but I’d be really surprised if it doesn’t support the Album Artist tag – you may just need to look for it.
Though you were able to edit things in Plex until it worked, I still recommend figuring out how to edit the tags in your files, for the reasons I mentioned in my last post.
In Plex, when you’re editing the Album, the “Artist” field is the Album Artist. As you found, when you’re editing a track, you see both the “Album Artist” and, down below, “Artist,” which would be the track artist. I can see why they didn’t label it “Album Artist” in the Album Edit dialog, but it’s probably a shorthand they could do without.
Plex didn’t intentionally make it difficult – I think they just need to more completely support the tags (as I’ve railed on about in this feature request). In addition to that, they could make the scanner more flexible by being prepared for “discnumber=1/2” or “tracknumber=4/12.” Those are valid tags per the specifications, and I think that’s how iTunes writes those tags. I know that’s how MusicBrainz Picard writes them. Plex needs the Album Artist tag to be populated – maybe iTunes doesn’t write them by default.
But with all that, I have found that Plex is pretty rock-solid at using embedded tags, if you have them exactly how Plex needs them. I think that’s true of any program – they need at least some reliable clues to identify albums and tracks, after all. You just have to follow the learning curve.
Anyway, I’m glad you got there, and I’m glad I could help. I really would recommend figuring out how to write the Album Artist tag in Kid3, though. You’ll be much better off if the info is directly in the files.
(You might take a look at Puddletag as an alternative to Kid3. I like it very much. That’s another learning curve, though 
).
