I'm curious - who is the PIC when an off-duty captain goes up to assist in a situation like this?
I was curious about this too. The JetBlue FAQ on the incident says the FO was PIC throughout.
What would have happened if there hadn’t been an off-duty Captain on the flight to step in?
Both Captains and First Officers are trained to fly our aircraft. Our First Officer was the pilot in command yesterday (in control of the aircraft from takeoff to landing) and the off-duty Captain stepped in to help.