If one were to ask Simon when he placed Merath under an illusion, the answer would be from the moment they stood in front of the crimson barrier.
When Simon told Sonya to not worry that he'd deal with Jorra and Merath if they tried anything funny, he had started dealing with the two Daemons from the moment he said those words.
He had been subtly using the Hollow Sky Eye to influence Merath's mind.
He chose Merath, not Jorra, for several reasons.
First of all, Jorra's mental defenses were sturdier than Merath's.
Jorra's abilities revolved around cursed spirits and cursed energy, but these were also related to his mental energy.
Simon was certain that for Jorra to be able to control cursed spirits, he needed to have a unique and powerful sea of consciousness.
So, he chose Jorra.
Simon had planned in his heart from the moment he started escaping the Shadowgrave Clan that he would kill either Jorra or Merath, or he would kill both.
