I understand that it's a complication situation. But then with compilation albums like
this, I'm sure that at least some official albums will merit the "featured" flag for artists.
There will be no sure way to know if a certain artist had direct involvement with an album in many cases, but we can make pretty rational assumptions based on the credits listed in an album, liner notes, and also whether or not there is new content based on that artist's role.
My thoughts on your examples:
Release on theme song single
- assuming this is the first version of the track, not "featured"
official soundtrack (short version)
- different version, but exact same artists listed, not "featured"
vocal album (different vocalist)
- is the composition the same or different? if the same, we might have the composer as featured, but assuming no arranger or new producer is listed, we could leave open the possibility that the original composer was involved (and if there are liner notes from the composer, that's a dead giveaway)
official arrange album
- if there is no new material from the composer, and there is no mention of him/her being involved in the liner notes, we could say they were featured, but this is subjective
best of compilation
- for a single artist, not featured; because otherwise, that would mean there is no non-featured artists assigned to the album, and that's generally not something that should happen -- a multi-artist compilation is a different story though
official soundtrack of sequel (instrumental)
- not sure what you mean
doujin arrange
- featured on
doujin remix
- featured on
remastered original soundtrack 2 decades later
- yes, this is a tricky one; assuming all of the artists are the same, I suppose this can be listed under discography, since these are kind of like alternate versions of the composers' original works, rather than derived works
Maybe we can use a rule of thumb where if, as in your cases, a "possibly featured" work is released specifically and officially for the same product as a new work by an artist, it can be included in that artist's main discography.