Greg responds...

He can die from prolonged exposure, but assuming the exposure isn't prolonged to the point of death, he can recover by having the exposure "removed". (Think Kryptonite, for lack of a better term.) So the answer to your first question is both.

Having said that, I did feel like we cheated a bit in Gathering II.

And I would think that if you managed to cut off Oberon's head with an iron sword, he would in fact die... assuming he didn't see it coming and removed his own head to dodge the blow.