To see the fields that are already in a file, see the extended tags dialogue (Alt-T).
To get a list of all supported fields see the help:
In respect to the fields that you mentioned:
ARTISTS is a user-defined field and will only be shown by players that support such fields.
Even though it is possible to create multi-value fields with MP3tag (several fields with the same name), such fields find hardly any support in players.
So the answer to 3: I don't know this depends on the player and its features.
The purpose of the sort fields is mainly to group names together that may vary like "Beatles" and "The Beatles". And as the purpose is grouping, it would be counterproductive to have several sort criteria.
On the problem of separation of artists, please see also this thread:
Noted. Thank you for your input. I was looking for the ultimate way to metadata my library and have it work if i plug it into any player in the future. That said, presently there is no player that support multiple things yet
For serious audio-ing: Foobar. There are so many extensions, more or less one for each purpose. And you can adapt the GUI so that it shows just that what you need in the way you like. It takes a little to get used to but from then on you never want to have another player (personal opinion).
Foobar is also available for Android.