Nope; the AI tends to only complete missions it would have undertaken anyways and betrayal isn't something you can normally get it to do.  That said, it's pretty slow to make alliances with the other AI's as well, so you should easily have "first dibs" on the ally of your choice.  In fact, the usual complaints are the opposite, that you can usually make an alliance and kill off all the other AI's before they can form any alliances amongst themselves.

I honestly can't speak for where the ball was dropped on the Diplomacy expansion, but most people generally agree it leaves much to be desired.  An improvement on the original game's lackluster Diplomacy, but not the polished awesome that starbases brought to Entrenchment.

I'd expect minor improvements at best.

while other games may have successful diplomacy systems, you have to admit that the situation with sins is quite different from those games. I'm not saying it couldnt be done; I believe it can and hope a proper patch/expansion/sequel will get it right. Sins is alot more combat focused, in my opinion, than those games.

If you can think of a game that has done diplomacy right, has alot of combat, and think the system could work for sins; I'd like to know what game that is. I'll try it out and get behind those recommendations.

rofls. total war... ya. war with everyone all the time. fun fun fun!