Falcon, If the transfer pumps had been on, then there should have been towards 76 kg of fuel in the supply tanks as the aircraft was over the city centre, a couple of minutes from the heliport, and about 70kg left on shutdown.
Fair enough, that's less than the operator's reported reserve of 85kg, but it's still over 30 minutes.
I think you're right that the time difference between the engines running dry is an open question, and could depend on the amount of fuel flowing from the right to left supply tanks based on attitude and acceleration in the minutes before the accident.
Late, tired, busy, startled, confused, alone… the very best could end up in error.