So if the assassin then decides not to attack and remains hidden, can he declare "Combat Over" and then restart combat hoping to roll higher the second time around?
As I stated, initiative would actually get rolled until stuff happens. That stuff could include the assassin taking an action, or the guard suddenly noticing the creepy guy hiding in the bushes right behind him, or another guard walking out of an outhouse just in time to notice the creepy dude, or really anything.