Just to pick up on a couple of queries I've seen in this thread.
1) Kodi fully supports a composer tag as well as a composersort tag. It also supports Conductor and Arranger for classical music
If you are using something like mp3tag, just enter each composer into the composer tag and separate them with a ; (semicolon)
Same with the composersort tag, so something like this for Der Stein der Weisen, oder die Zauberinsel
COMPOSER Johann Baptist Henneberg;Benedikt Schack;Franz Xaver Gerl;Emanuel Schikaneder;Wolfgang Amadeus Mozart
COMPOSERSORT Henneberg, Johann Baptist;Schack, Benedict;Gerl, Xaver Franz;Schikander, Emanuel;Mozart, Wolfgang Amadeus
Bold bit is the name of the tag, next bit is the values.
Depending on a setting, when navigating by artist names, or composers, lyricists etc kodi can either sort on the first name or last name (if composersort was set in the tags) - most tagging software does it automatically. Picard definitely does.
For instance, with the setting disabled, Wolfgang Amadeus Mozart would appear in the "W" section. With it enabled however, he would be in with the "M" section.
DISCART
naming this depends how you have structured your files. If you have
Artist
|---Album
|-----Disc 1
| |---- tracks
|----Disc 2
| |---- tracks
Etc, then each disc should be named discart.png and be in the disc directory that it belongs to.
If however you have all the tracks for a multi-disc album in one directory, possibly named 1-01-song1 1-02-song2 etc where the first number is the disc number, then you should put all the discart in the same directory as the songs and name it discart1.png, discart2.png etc. Kodi will find it and apply it to the right set of tracks,
On the subject of multi-disc sets, you don't need to actually name them to get Kodi to recognize a boxset. Although they usually are named, if there are 3 or more discs to an album and the album is not a compilation then Kodi will auto convert it into a boxset and create "fake" names of "disc 1", "disc 2" etc when adding it to it's library.
Its actually really powerful once you get to grips with it, but I've been using it since 2012 and I occasionally still find stuff I didn't know. And that's without living on the bleeding edge...!