Well I figured out most of the questions on my own.
I did the power outage test. I marked the sump for my max water line while still allowing for drainage water. Then I moved that line an inch lower just to be safe.
I also did the siphon-breakage test. I drilled holes in the tube return line about a quarter inch below the water line. When it is broken it fills up my tank to the brim, but no overflowage and the reverse siphon is broken by the holes. I now know the maximum fill levels for my tank and sump.. pretty cool stuff.
I drilled a hole in the top of the overflow box u-tube. I put airline tubing in that hole and then connected the other end to my power head. No air in there!
The heater is fine, it doesn't touch the containers so it won't melt them.
So.. useless without pictures! Here's my (almost) final sump and i'm proud of it!
Full sump, in action
The water pouring in the last container and my max fill line marked with tape
The overflow box u-tube with airline tubing in it (there's no air in there, that is silicone on the outside of the tube)
Now, to figure out what to put in my middle sump container...