How does that scenario work? Someone's under duress because presumably there's a terrorist on board. He lets the crew know there's a bomb onboard. The plane turns around, and the terrorist... lets the plane land safely?

OK maybe the bomb blows up when it crosses some longitude, because this is like the movie Speed, and turning around means the plane never cross that longitude..

If you mean another type of duress, naming your device "plshelp-[seat number]" would be a hell lot more effective..

Honestly I didn't think about that. Maybe they didn't either. Good example of why seeing something vaguely threatening and out of the ordinary is a reason to turn around, even if you don't know why exactly they'd do it.