Re: Detaining pax on board - Legalities?
Trip, I am not a lawyer or 'plod'
If smoking on an aircraft is a criminal offence and you had reasonable suspicion, eg smokers breath, independent witness, that the individual had committed the offence then a 'citizens arrest' as described would be lawful. Leave proof to the Courts
If an offence under Civil Law, unlikely, then no power to detain
Re other pax, you only requested their co-operation. If a pax had insisted on disembarkation (to make a connecting flight etc) I suggest all you could do would be to satisfy yourself you had the pax name, contact details for the Police to follow up, if the pax was not implicated in the offence.
If the offence suggested a conspiracy, then all pax & crew could be considered suspects and wholesale ground detention would be lawful subject to 'reaonable' force, provided you do not end up with a hostage situation
I use the term 'reasonable' in the legal sense - if physically aggressive, restrain; if he points a gun, kill him if no other option