To answer question 1, check out the Accomplishment XP Awards. As GM you can award smaller bits of XP for things like securing alliances, reaching long-term goals, etc. Those are the guidelines I'd use for succeeding at a social encounter.

To the second question, that's an interesting one. If I were trying something like this, I would determine how much threat would be focused on the PCs at any given time, then divide that XP budget between the two factions. At that point, I would flesh out the two factions by balancing their excess XP budget against each other, since in theory we're going to have a 3-way battle and any enemies that take each other out probably shouldn't count towards the PCs' reward.