Friday nights would be the worst. People get off work, and go party. On Saturday night, they're hung over from the previous night and just hang out. If you visit restaurants in your local area, you'll notice they are packed on a Friday night, and on Saturday night, it's better.
As to why they close IOA an hour early, they just do, and that's the way it was last year. So the best idea is to head over to the USF side, ride the Mummy just before closing, then go to the Stay and Scream area, where the first two houses will be available. You can walk thru them in minutes, then you end up waiting for them to finish clearing out the USF side.