Harry figures out what Maggie's death curse was based on her gift of intuition in the soul gaze, so I'd say that it isn't confirmed who the curse draws power from. I think it's beyond any reasonable doubt that it feeds off of Thomas and Harry. I also believe it's more likely than not that it draws power from little Maggie. It could have been severed from Harry when he "died" any one of the 2-3 times he died. And it could be blocked by Demonreach from using Thomas. So it's possible that the death curse is broken/inactive as of the end of PT. I don't think it's likely, but we've never been given precise details about how anything works, so Jim almost always has enough wiggle room to do what he wants.
