Additionally, do Independent Provinces ever venture forth and attack you? So, if I leave one alone next to my province, do I ever need to worry about it?
nope, indes don't attack, except as random events - and then its a new army generated for the event.
an interesting bit of trivia - if you beat an enemy via banishment in the death match (such as sending them to the inferno) they will come back next round, etc, until they are either dead or you are.