1. Good question. The UNSC takes care of their own and if a Spartan decides to retire I'm sure the UNSC will make sure they live in a comfortable position with a stead income. I doubt they'd just forget about them after all they've done. I'm not sure if they get paid during the years they are active as I doubt with their usefulness will get a few months shore leave every once in a while.
2. The only Spartan to have retired is Maria and it's stated she wanted to try to start a family however we don't know if her reasons for retiring is just that or if she had some sort of injury that forced her to retire. I doubt any Spartan that can still operate Mjolnir and fight would retire. Spartans like John, Kelly and would rather fight until they die.
3. Running underground Grunt Fighting Rings. Place your bets, give each grunt a paddle, get drunk off your ass, enjoy the show. It's either that or they'd likely do non combat roles in the UNSC like training marines, test weapons, work in navigation, become a physicist or something like that. They are technically geniuses after all.
[Edited on 08.17.2011 10:45 PM PDT]