Alright, I doubt this will get much support but it'd be realistic. I think that posts in the rpg board should be editable by everyone. This way your character can mar up other's posts, shred them, ect. Of course it'd be hard to distingiush it from the normal rpg posts that are story based (as in it'd be hard to not let others mod these), but I think this would be good idea if possible. *awaits flames*
It would be abused to hell. That's all I really have to say. Perhaps if you kindly ask the poster to edit the post themselves, they might, but I can't really think of too many ways this would be needed.
I think we can depend on the good roleplayers to re-edit and "mess up" their own posts, so long as people don't go around messing those messages up to an excessive amount.
I don`t know the reason for including something like this. The situation to have to change/delete parts of an other post is too rare. If you want to change anything in the post of an other, you could also ask the author.