Related to this, what is the deal with open threads, because if anyone participating in a thread can attack, then ten characters could join an open thread in quick succession and attack anyone already involved in the thread?

You get 50% more exp for open threads, so I think the risk of having someone join and slap you around seems fair ;)
ETA:
Do attacks in RP count towards a character's hit limit on the grid? For example, if someone is attacked twice in an RP thread, can they only be hit another couple of times on the grid during the same day?