1. The AI is as useless as most strategy games, but SP can still be fun; especially as a testing ground for nation/pretender setups.
2. It is. Perhaps too deep
3. email games are done, but probably more common is to have a server running the game on quickhost - so, when the last person does his/her turn, the game hosts and it is a new turn. Usually this is done w/ a time limit, ie. if someone hasn't done his turn w/in 24 hrs of the beginning of the turn, it hosts anyways (this would be "24 hr quickhost"). As well, there are blitz games organized through IRC, where either the whole of a small game is played at one sitting (or at least until a suitible endpoint), or else at least all the initial part of a game.
4. There's no answer to that. it depends on your nation and the situation.