how are they going to run two engines whilst everything is going on during the turnaround
I don't think this question has ever really been answered.
Those of us who fly for a living- and have been doing it long enough- will have come across a circumstance where the combination of ambient conditions and lack of APU bleed air for aircon AND no ground air available will result in a very hot cabin. I've seen 37 in a cabin with no pax and the flight deck was hotter but alas, no temp gauge. With no air and full pax, the figure I've always worked with is a degree every two minutes at 25 degrees. 10 minutes with no air con and a full load the cabin will have gone from 23 to 28 degrees. 10 minutes later it'll be 33. It will eventually flatten out but once it's above 33 degrees, who cares if it flattens out at 37 or 39, it's still freaking uncomfortable.
So the solution on departure when you don't have air for packs is to load up the aircraft in terms of cargo and catering and then once everything is done, start an engine(s) on the gate to provide air to the air conditioning packs. Once the aircraft has been cooled enough to make it bearable you then load the punters and depart. It normally results in significant delays as you can't begin to board until the aircraft has cooled reasonably and you can't start that normally until at least departure time when all the 'normal' stuff has been done and all the equipment has been cleared from the aircraft. Cooling takes a bit longer than warming- 1 degree every 5 minutes @ 25 degrees rings a bell but it's been a while- and so you're looking at 15 minutes to drag the cabin from 33 to 30 in order to then board. Tack on the 20-25 minutes to board a 767 domestically and straight up you're looking at a 40+ minute delay.
I hope you appreciate the answer VBPCGUY. I wonder if you'll admit you were a bit presumptuous and jumped the gun in suggesting something wasn't done correctly?