(begrudgingly) resolved

I took care of the freezing fullscreen issue by downgrading to Flash Player 9, which was a considerable bother to find online. I played some of Hulu's HD content, and it was gorgeous and not choppy. That's the first time I've rendered HD content – quite a relief that it works.

I don't know the difference between Flash Player 9 and 10. The HD content plays fine, so I don't think I have a reason to care. (It still bothers me on principle!)

I also took care of the fact that my computer goes to sleep when watching videos.

#!/bin/sh
trap 'pmset force -c sleep 90' 1 2 3 14 15
pmset force -c sleep 0
sleep 7200
pmset force -c sleep 90

I put a link to this file on my desktop. I just need to double-click it (thanks to its .command suffix) to disable the inactivity-based-sleep functionality for 2 hours. Break it down:

1. The trap means that if the script gets prematurely interrupted, it re-enables the sleeping.
2. The pmset bits disable and re-enable the sleeping behavior.
3. It re-enables automatic-sleepage (... eww) after a 7200s=2h delay via sleep.