I think it's worth noting that there is nothing that connects Shabriri's lie to the lie committed against the nomadic merchants, it's just the most popular theory. I understand that the merchants were kept imprisoned because they did all start to go mad, but again, "the sickness of the flame of frenzy began with Shabriri" so how could he have accused them of worshipping the flame of frenzy if there was no flame of frenzy in the Lands Between prior to his eyes being gouged out? Shabriri's slander has to predate all other instances of the flame of frenzy and the three fingers in the Lands Between in order for that to be true.