As I feared, these files are completely un-tagged.
Here is a free tagging software which also works on OSX
http://kid3.sourceforge.net/
For a serious ‘deadhead’ who collects a ton of bootlegs, you should give embedded metatags preference over data from last.fm, since these bootlegs are rarely listed:
move the ‘Local Media Assets’ agent to the top of the list under
Settings - Server - Agents - Albums - Last.fm
This file and folder naming will work:
Music / <-- music library points here
Grateful Dead /
artist-poster.jpg <--- a photo of the artist
artist-background.jpg <--- a background/wallpaper particular to the artist
Niagara Falls 1984-04-17 (gd84-04-17.137250.sbd.miller.flac1648) /
folder.jpg <-- if you find a suitable album cover picture you can add it here
background.jpg <--- album background/wallpaper [overrides the wallpaper from Artist]
01. Grateful Dead - Tuning.flac
01. Grateful Dead - Tuning.txt <-- "pure text" song lyrics
02. Grateful Dead - Jack Straw.flac
02. Grateful Dead - Jack Straw.lrc <-- "timed"/synchronized song lyrics
...
21. Grateful Dead - Encore Break.flac
22. Grateful Dead - Touch Of Grey.flac
The renaming and tagging was done in under 2 minutes, due to the powerful features of my tagging software which can derive tags from filenames and vice versa.
Embedded meta tags should be set as following:
AlbumArtist Grateful Dead ← AlbumArtist is a different tag than ‘Artist’!
AlbumTitle Niagara Falls 1984-04-17 ← if you want to include more ‘bootlegger’ info, just do
Disc # ← not necessary for a single-disc album
Track# 1 to 22 for each track accordingly
TrackTitle <— self explanatory 
Artist Grateful Dead
Release Year 1984
This should work.
After changing file names and embedded metatags, perform the Plex Dance (with music you can omit step 4)
This will remove the mis-matching from the Plex database from your previous attempts.
I renamed and tagged the files as described above and added them into my last.fm library.
This was the result (I didn’t edit any metadata in Plex):
