I don't have fraps to confirm this, but I am almost positive you are correct and that they spawn based on his life. At least what we did before we got geared is DPS the boss until the second set of adds came, then DPS only the adds until they all died. No more adds come during that time. Once all the adds are dead, and the healer has energy, we resumed DPSing the boss and kept on him until he died, ignoring the adds that spawn at that point.
Not sure about that. Could be a hybrid trigger, though I'm not sure they have those in this game. I've definitely seen way more than 4 groups before. So maybe there is a timer which spawns and resets every certain amount of hp?