Good questions. Some of us, here in MC's Discus, have been discussing what would happen if mana disappeared from this world? Right now, the giant DK is disappearing so no more mana source from the First Tower.
However, we do have to consider that there are other Towers. You have to wonder what's in those Towers? So, I don't think magic/mana is going to disappear so soon. (Lispen looks like he knows more than what appears to our very eyes, so, the battle is far from over.)
Rood's DK powers will remain because he doesn't need to rely on outside mana anymore.
And we don't know what Meredith is. :D