1. Since I use FLAC for archiving I only have mixes/livesets with splitted files (which play gapless), which are all tagged separately. In the past (old MP3 times) I tagged title with set/mix name and artist with mixing DJ.
2 & 3. Let's say I've a song "Lena X Nico Santos - Better (MOKABY X Blondee &. Roberto Mozza Remix)", I go this way. Display and sort artist tag: "Lena X Nico Santos", then separate artists, artist:Lena, artist: Nico Santos, then remixer: MOKABY, remixer: Blondee, remixer: Roberto Mozza. Title tag: Better (MOKABY X Blondee &. Roberto Mozza Remix).
If I have a feat. in title, I set a guest artist tag and write the featuring artist into display and sort artist tag. I. E.
"David Guetta -
Can't You Feel The Change (featuring Chris Willis)", I'll tag display/sort artist: David Guetta featuring Chris Willis. Artist: David Guetta, guest artist: Chris Willis. Title tag: Can't You Feel The Change. Can't show you pictures yet, but hope it's understandable.