Are we even certain what type of poison was used against Quain? Cause its definitely not the madness inducing variant Snad handed out. We saw what happened to Zala when only scratched with the stuff (remember she had "padded" belly protecting her from most of the daggers thrust it would have also served to absorb the poison on the blade).
I am kinda beginning to suspect given how slowly it was taking to kill Quain and the possibility most of the damage to her face was being done by the healers in order to get rid of it that it wasn't a poison at all. More like some sort of acidic substance. Would explain the slow dissolving of her face. Rather than the Poison actually getting into her blood stream and actually properly killing her in a timely manner it seemed kinda content to do damage locally.
Would love hearing some thoughts regarding this cause honestly the effects of it leaves me with more questions than answers.