Not at all because the amount of exp is fixed. Some quests though will only give exp to one character.
Yes they do. What other things?
Don't know.
In single player, ALL XP is based on your main character. Other characters don't even have XP bars, they just level up when the main character does.
Anyway, I had level 21 at the end. I imagine if I killed everything (ie civilians and guards) the max could not be higher than 23. The exponential increase in XP per level means that killing everything and completing every quest in the first two acts is largely irrelevant to max level compared to killing everything in Arx and the Nameless Isle. 22 seems very doable, 23 is probably pushing it.
Of course, there COULD be an as-yet unknown exploit allowing infinite XP. Those things always tend to slip through somewhere. I don't think there is a hard cap at all. Everything is scaling-based and there's the whole GM mode to consider.
