I think wow is pretty consistent that the light countering undeath in all kinds of ways and that when the LK is weakened undead can break free like we see in WC3. As such the light just weakening the lich king if in huge amounts instead of any of the more dramatic effects fits fine and it’s the LK being weakened that lets them break free not the light it self.
As far KT and Dorian, the light doesn’t stop some one from being raised that’s why the LK wanted to take lights hope because of all of those delicious holy men to twist and raise. Dorian trades his soul into undeath to free his fathers and in theory if it wasn’t for KT scooping him up he shouldn’t have had any one to take control of him and he’d have been free from the start.
Also ya the Lich king should just be able to retake over any undead that get near him the idea that they can break free when he’s weak really doesn’t cover why he can’t grab them again.