Thank you for the information to check the relay circuits. That answers part of my question, the other part is what steps do I take to check that the cooling system is intact and what else do I look at if it overhears again?
I am worried that there may be a head gasket leak, but A) I'm not, or at least have not been losing coolant (I hadn't needed to top up the overflow reservoir for more than 6 months of daily driving, and the rad was still full when I flushed it), B)the oil is clean and there are no signs of cross-contamination, and C) the exhaust is occasionally white upon startup but not noticeable at any other time (seems to be mostly related to atmospheric conditions).
I did try looking at the coolant in the rad with the engine under strain, and there were a few bubbles, but it seems inconclusive. I know I can do a leak-down test, but I'd rather test everything else before resorting to this (these minivans are not the most convenient vehicles to do engine work on, even pulling the plugs is a hassle.)
Before the rad flush, I had occasionally had problems with overheating on the highway. Usually I'd pull over and turn the heater on inside the car and it would cool down. However, I'm wondering, could this also be related to the water pump? How do I know/check? (The hoses don't seem to be running "full", but they are running hot, and under most conditions the temperature gauge stays around normal operating temps.) As mentioned previously, I did try to empty any air pockets by using the bleed valve near where the top hose connects to the thermostat inlet. (It didn't seem to work at all, though if I squeezed the top hose with the bleed valve open a small amount of coolant would come out.)
I know, there are really many questions. I just really want to make sure this van is working because I was planning to use it to move to a different province later this summer. After that, I'm ok if it goes to the scrap yard, as I have already purchased another vehicle, but I'd like to preserve it's life until at least August.