Under what circumstances can a flight attendant unbuckle and get up during landing?
An FA is a member of the crew and can get up or sit down as they wish during any phase of the flight. As to what they can do, they can have the pax arrested the moment the aircraft door opens. If that is not a good reason to obey instructions, I don't know what is! If the pax becomes a threat to the safety of the flight, yes, they can certainly get physical to put an end to the situation and secure the cabin.