I like splitting stone. I also think there should be a geomancer spell that creates a high piece of terrain you can stand on for height advantage, assuming that's possible with the engine, or at least as a source of cover.
Sacrificial Gift sounds like a good way to make the other necromancy skills more useful since you can use your health for something else. Possibly it could grant some additional buff with the healing if you're really sacrificing 50% of your health.
I don't think Shackles of Pain needs to be changed. But there could be a whole separate skill that does what you suggest.
