Charlie98:
Yes, the 757 has 'gasper' vents overhead as well as vent panels on the floor.
The APU will need to be running with at least one air conditioning pack on - but if you're somewhere warm, it will be.
They are switched off during engine start as the air is required to spin the engines up. Always makes me smile when I'm sat in the back and all the pax start fiddling with the gaspers during engine start and start complaining (whiney voice)'my blower doesn't work....'
As for a 16 hr delay, you'd be extremely unlucky. I can assure you if it starts getting towards a 2 hr delay that looks like it could go to 3 hrs, the airline will be doing everything it can. Of course I should say, in fairness, the airlines do try hard not to have any delays at all........