Will changing the setting on my Crock Pot reset the timer?
From experience cooking chili in Crock-Pot brand slow cookers, each time you change the setting (at least from high to low or vice versa), it resets the timer. Also, of course, so does high->off->high.

So, (at least my Crock-Pots) would cook on low until midnight in your example.

Well, unless the power went out for three seconds. (It'd be off after the power blip.)

Personally, I've switched to non-"programmable" slow cookers.