Can they say they will tell somebody as soon as they notice strain?

That doesn't really sound like how demons work. Does it, Nenio?

...Sure, they can say that.

"Some types of demon, the most common being shadow demons, do have the ability to possess mortals! However, it would be a highly unusual coincidence for the possession to end less than a minute after the possessed subject attempted to stab another individual, so that explanation seems highly improbable. Furthermore, most forms of demonic possession can be detected with Detect Evil, unless the possessor has a way to conceal their alignment aura."

There's paladins around - if you're trying to use a Zone efficiently like this you need multiple people on hearing the affirmations of separate streams of participants - did they notice anything?

They did not detect Evil near him, no.

So yeah, sounds fake, he will get a trial but it's not going to be a particularly complicated one.

When he sees they're not buying it he will even helpfully switch to telling them all they are powerless fools caught in Baphomet's maze and too stupid to see it!

See, didn't have to specifically aim the enterprise at cultists to flush one out.

All told, there are a little over a hundred people unwilling or unable to affirm the statement, not counting the people he worked something out with, though presumably some of the remainder simply managed to throw off the spell. (There's a sense in which this is good news! Most of his advisors thought it was going to be higher!)