If you ask me, Sylvanas went off the deep end back in legion when she tried to enslave the Valkyr in stormheim, she just kept diving and diving after that.
As for Uther’s souls split I’m more convinced that the Light saved part of his soul while Frostmorne took the other part. The part that was saved was the typical yellow color we typically associate with the light in WoW and he asked the light to save his soul as he died. Not to mention Uther has nothing to do with Azerite.