I thought the original idea was just to avoid the nukes flying?
If there are difficulties, then the other diplomats' tolerance for mistakes is vastly reduced.
"You performed the greeting of my people in the wrong order! It is handshake then bow, you uneducated cur! You must be annihilated!"
Other objectives should be optional, with a range of difficulties, but increase your score or something. I'm not sure about the idea of assassination, though. Like HugoLuman said, the premise of the game is that people go mental at slightest mistake, so murder just seems a bit pointless. Unless there's a separate mode in which the idea is you're an assassin and have to remove someone (or more) without it being discovered, and escape.
Some suggestions as to traits:
Isolationism would sound better as openness, their willigness to discuss things with other diplomats. While isolationism is the name of a political policy basically isolating yourself from other nations (North Korea these days, for example, or the US between the world wars, or Japan before the Europeans arrived) I'm not sure it can really be applied to people. That is, of course, unless we're using these as national traits as opposed to describing the individual diplomat.
Serenity should be patience or tolerance, though should be exactly as you describe (their quickness to anger if you do something wrong)
Influence sounds perfect though.
If there's likely to be alcohol involved in the game, perhaps alcohol tolerance should be another trait, basically how much alcohol they can consume before they get drunk, and if they keep drinking, how much more drunk they can get. Also a random chance of the kind of drunk they can be (aggressive, depressive, happy, lusty etc.) which can influence how they interact with others.
Are there likely to also be relationships between the different npc diplomats? Not sure how much effort you want to put in at the moment, but if you're getting on well with diplomat A, and diplomat B doesn't like diplomat A, you find it harder to get on diplomat B's good side, kind-of-thing.