Here's another newbie's take on some answers. I'm an extremely relevant newbie, though...
2) making a non-aggression pact is mostly meaningless unless you also agree to some minimum length, whether that be "until turn 25" or "until I give 3 turns' notice" or whatever. It should be explicitly stated, if the pact is important to you. I bet they do get broken, but I bet it doesn't happen all that often, and (touching on 6) when it does happen the rat-bastard loses a lot of trustworthiness in the eyes of anyone who's paying attention.
3) I don't see why it would be a responsibility
4) Sure it is. It's not very polite, but it's not dishonorable.
6) If someone backstabs you, it's fair to regard the person with a healthy amount of distrust. If someone plays a country that has a big nasty war with you, there's no reason to hold a grudge, as that's what the game's about, in the end.
7) Ah, the free-market economy...
8) Depends a whole heck of a lot on the situation
9) The later the better, up until the point where you clearly can't put up any effective resistance at all (at which point, especially if the game's on quickhost, there's only academic interest and pride to keep you in)
10) I dunno, but I hope it's not too common...
In general, I think people expect others to behave decently and honorably and honestly and friendlyly interpersonally, but not necessarily in-game. If I make a deal with you, I'm bound to follow through, and expect you to do the same. Of course, I'm probably planning to scorch your lands and sacrifice your virgins in-game, but I have to do that without violating any of the standards that we as humans have when dealing with each other.
Cheers!