To cover something from above... In the barbarian section it says that Barb XP stops at "level 3 and 30 xp". I think they mean to put an "or" there (otherwise it doesn't make much sense to me). It also matches with the system of Civ IV where you'd stop getting barb xp at level 3. Either way, extrapolating from that could (likely) mean 30xp=level 3. We then have the possibility that it uses the Civ IV progression multiplied by 3.
6/15/30/51/78/111/150/etc.
United front seems pretty clear: All other players lose influence with city states 33% faster. Influence obviously has thresholds for friendship/alliance, but even if I'm allied with Monaco, Napoleon can still have influence/be friends. If multiple players have united front, then it will be cumulative, like other percentages. (66% faster if two other players have it, etc).
That is how I see it. The influence info is below...
-60 to -1 = Angry (no benefit/penalty. War is "always -60")
0 to 29 = Neutral (no benefit/penalty.)
30 to 59+ = Friends (Open Borders / Half the rate of their bonus)
60 or more = Ally (
only if you're the highest influence holder, Open Borders, full bonuses, access to any resources they improve)
How many social trees can you have active at any time?
You can only work on one at a time from what I've seen of 2KGreg's gameplay (there were little locks on the other branches), but you can still get benefits. I don't know what it entails to change your policy focus (one Honor to get Discipline [+15% strength for units in a tile next to others] and then one honor to get Citizenship [+50% Worker production]), but based on the warning in the book regarding policy conflicts (see below), it would appear that you keep the benefit of policy chains which aren't your current.
Conflicts are..
Religion / Rationalism
Autocracy / Freedom
The warning is...
"You can't get benefits from XXX and XXX at the same time.
If you invest in one and then want to switch to the others you must endure a few turns of Anarchy. Also, you end up wasting Policies that were taken in the branch that you've abandoned."
So... not 100% sure you can have multiples active, but based on what I'm reading you don't LOSE the policies you WASTE the policies (ergo you still have them but you cannot use them due to the conflict.) That said, you could have 8 entire policy chains active at one time unless you are Autocratic in which case you cannot have Freedom OR Liberty (though strangely there is no warning under Liberty).