Remove friendly fire outside siege zones. Allies, and shouldnt be able to attack eachother.
Its good so they can in sieges. That way its more fun.
Outside sieges, theres pvp plots.

Now to occupation pressure:
Occupied nations should appear as allies to the occupier/overlord. They can not pvptag the occupier nation's members, but the occupiers can. It would work like the sheriff role inside towns.