I don't know the exact formula, but I can answer some of those questions:
- Team record doesn't count much. Last season I finished 22-0 and won the championship but didn't have a single (!) player in the MVP voting
- Overall stats count more than stats per minute. My starters averaged less than 30 min per game, had good stats per minute but weren't among the leaders in stats per game.
- Considering a player that left the league, I would guess that, as long as he had played the 15 games required to figure in the stats he would be eligible for MVP voting. But I may be wrong