I do not believe it can be purified even in its base state. It would be like attempting to purify any other type of energy, power - so you are correct. The absorption cannot be undone.

That is a good question. In my own world, gods are nearly impossible to kill, but this one behaves with its own rules. As it is, you're correct to wonder if it would even work to stop the dark water. We know it can still affect the corpses of mortals after all. Why should the chained gods corpse be any different?