Strange. I thought Ishamael claimed that one of LTT's "sisters" could give him lucidity, if only for a moment. Though maybe OP can be used to help with the symptoms, but TP can heal it completely. Or maybe Ishy was just talking nonsense.

It does make sense though, Taint is the DO's power, much like TP. And the wound on Rand's side inflicted by the TP is apparently unhealable by OP.

I wonder if Moridin could heal the wound he inflicted using the TP. If it gets worse and Rand starts to die, Moridin might not have any other choice since he thinks that due to the link, Rands death will kill him too.