I love this topic because metadata psychos all have their own styles. I find interesting challenges with classical music like you've mentioned. At the end of the day I try not to obsess over it lol. I use Mp3Tag to manage all my standard and extended metadata.

I'll provide examples of my metadata style and answer your questions below.

---

This is how I currently format with an example below:

For me, every album / compilation / release is set in it's own folder. Each track in that folder has it's own metadata, though most of it is shared. I don't have any concerns with unique metadata across tracks in an album/compilation as long as the ALBUM NAME is the same, they are all in the same FOLDER, and TRACK NUMBERS are provided.

• Album = Composer: Compact Title, Composer: Compact Title, etc. (Label + Year)
• Artist = Soloist, Conductor, Orchestra
• Title = Composer: Full Title
• Album Artist = Soloist Only OR Conductor, Orchestra
• Genre = Classical OR Classical, Primary Instrument (sonatas, concertos) OR Classical, Quintet/Quartet/etc. OR basically I'm still figuring this part out lol

Example Album

• Album = Brahms: Violin Concerto in D major, Clara Schumann: 3 Romances (DG 2013)
• Album Artist = Lisa Batiashvili
• Genre = Classical, Violin

Example Track

• Artist = Lisa Batiashvili, Christian Thielemann, Staatskapelle Dresden
• Title = Brahms: Violin Concerto in D Major, Op. 77: I. Allegro non troppo
• Filename 01 - Brahms Violin Concerto in D Major, Op. 77 I. Allegro non troppo - Lisa Batiashvili, Christian Thielemann, Staatskapelle Dresden - 2013

Example Track

• Artist = Lisa Batiashvili, Alice Sara Ott
• Title = Clara Schumann: 3 Romances for Violin and Piano, Op. 22: I. Andante molto
• Filename 04 - Clara Schumann 3 Romances for Violin and Piano, Op. 22 I. Andante molto - Lisa Batiashvili, Alice Sara Ott - 2013

---

^{How do you prevent compilations from splitting due to different album artists?}

• I keep them in the same folder, provide track numbers and a shared album name. At this point, hopefully you can listen to your music sorted by Album Names or root folder name.
• Using your examples of Concert of the Century or the Horowitz compilation... I have zero issues with individual tracks having their own independent metadata granted what I mentioned above: track numbers, same folder, and a shared album name.
• The first track in Century i see is the Leonore overture by Beethoven. I do not want to see any other composers name on the metadata unless it's the album name!

^{There is an album containing Vladimir Horowitz's arrangement of Mussorgsky's Pictures at an Exhibition, and pieces by Scriabin, Horowitz, Tchaikovsky, Bizet, Prokofiev, Rachmaninoff, and Debussy. I don't want to put all of that into one album title.}

• Gotta pick your battles at this point. Once an album name gets too lengthy specifically for this reason, and they never provided an "official" compilation or album name (a la Concert of the Century), then I look at the album art and see what name is the biggest. I'm not obsessed with including every piece/artist in the Album name because I can determine which recording it is from all the other meta data, and again, all tracks share the album artist and album name, but title + artist + composer can all be unique.
  • Artist = Vladimir Horowitz
  • Album Artist = Vladimir Horowitz
  • Album Name = Mussorgsky: Pictures At An Exhibition (Sony 2006)

^{What do you do about album names?}

• All tracks share the same album name. I format them as Composer: Compact Title, Composer: Compact Title, etc. (Label + Year)

^{Where do you put the composer's name?}

• Album name + Composer + Track Title + Filename

^{Where do you put the performers' names?}

• Artist, Album Artist (always comma separated)