The mesmerist can trigger this trick when the subject is affected by a charm or compulsion effect created by an enemy. The subject acts as though the mesmerist had cast the spell instead of the enemy.
When the mesmerist successfully affects the target of his hypnotic stare with an enchantment spell that requires a saving throw...
How do these two abilities interact?
Rule Minds activates when the mesmerist affects the target with an enchantment spell while they are affected by hypnotic stare.
Mental Fallback makes it as though the mesmerist cast the charm or compulsion spell (which are subschools of enchantment).
Does mental fallback count as the mesmerist casting a spell in the context of Rule Minds?
For the most part, this has little to no practical value, of course. If you are going to the trouble of putting a trick on an opponent, and then casting an enchantment spell at them, you might as well cut out the first step and just get Rule Minds Normally. And using Rule Minds has...limited use on allies, particularly since you might have already highjacked a domiante spell.
No, the reason I ask this is simple: overly complex subterfuge, meant for scheming villains. Because...that is one of the things mesmerists are best at, no? So the idea here is that the mesmerist woud place themselves as 'loyal servants', offering their various abilities. But all the while, they would wait for the chance to steal the mind of the target without fearing reprisal or failure.
Basically, I want to see if this is the answer to "how to defeat a level 20 wizard in his lair"- spend 20 years serving him loyally, all the while waiting for the chance to steal his mind and magical might. I just think it sounds delightfully convoluted, and brings the obvious advantage of having him switch to be the BBEG while gleefully thanking the heroes for giving him this chance.