I'll nit-pick this one
the PIC would check with emergency services before ordering an evacuation
most circumstances a prudent Captain will liaise with emergency services, but there are many circumstances where, still at the gate, with the aerobridge still attached, a prudent captain will call the evacuation and inform the emergency services that the Evac is underway.
This slightly weird conversation about who has the legal responsibility for the pax safety while on the gate is quite interesting / important though. In the United case my guess would be that the captain had the legal responsibility and the security guard then acted illegally in using excessive force. Like I said on another thread I think that in the future I will be having a 2minute chat with the 'police/ security ' prior to them removing someone in case they happen to be the 1/1000000 nut job like in the United case.
My long held assumption is that prior to the captain arriving at the aircraft and boarding the ground engineer would be responsible for the safety of anyone onboard should a fire or other emergency occur, in a perfect world the engineer would 'hand over' to the captain and the responsibility would transfer. Now days with the engineer off on another gate it is less clear what the situation is. Hopefully through this conversation we might nail down something