1. rebels are effectivly 1 faction in the game so they can withdraw to friendly territory.
2. Never seen it
3. Have seen this, can't really explain it, but the only time i've had it happen to me i think was when i had been ex-commed and was re-communicated, possibly in civil war if you kill the rebel leader you will get all their lands back.
4. What's wrong with that? They do rarely attack, but if you are at war with rebels they'll sometimes do this.