The Cishaurim believe that their sorcery is the Water of Indara, but Kelhous's explanation indicates that's not how their sorcery functions.
At this point, either/neither explanation is still possibly false?
Sure. We don't know for sure. Kelhous's explanation is logically consistent and Titirga's experience seems to support it, so it's likely to be mostly accurate, but we don't know for sure.
instead cured of his blindness
Always wondered about this. I like locke's thought about vitamin deficiency but I do wonder if there isn't another in-world context that explains it.
Sorcery seems to be pretty useless for healing (except fpr cauterizing wounds) and vitamin deficiency blindness (as opposed to night blindness) appears to be permanent. Cortical blindness, which can be congenital, is caused by blood flow problems in the brain and some victims do recover their sight over time. So it is possible Titirga suffered from this, regained his sight, and then was train by Schoolmen. A brain injury might also explain his fearsome rages.