RMC (and SWBKCB),
interesting subject. I checked with an authoritative source: my recently-retired F/A wife (52 years on the job...I'm a slacker at only 35).
She says that:
(1) There's no enforcement of the seat belt sign with the airplane parked at the gate with the cabin door open. At the point the agent comes to close the door and be told the "cabin is ready for push back", you must have the belt fastened.
(2) Yes, you can get off the airplane if you insist. There may be some complications for an international departure but there are procedures for that.
(3) There's little likelihood of someone sneaking off as one F/A is assigned to hover around the door from the start of boarding until it's closed. Anyone walking off without explanation would be seen.
(4) You can bet checked luggage will be pulled when a pax gets off.
(5) Is it possible the system can fail ? Yep, it's a system designed and operated by humans. But the industry makes all reasonable efforts to operate securely.