Thanks to all those who replied. I believe I now have an explanation as to why the overflow reservoir starts boiling over at the moment the engine is turned off. My theory is that there is a pocket of coolant inside the engine that is overheated, but as long as the pump is on, it remains pressurized to the point where it cannot boil. The moment the pump turns off, the coolant pressure equalizes throughout the system...the overheated pocket instantly boils, forcing the system pressure to rise above the release point of the radiator cap. The coolant which floods into the reservoir, now at atmospheric pressure, begins to boil as well.
According to my theory, then...there is boiling water deep within the engine at a pressure HIGHER than the 9psi release point of my radiator cap...there is somewhat cooler boiling water in the overflow reservoir at atmospheric pressure...and there is water in the radiator, slightly above 212 degrees, which is sitting at 9 psi and NOT boiling.
One thing that ISN'T helping the situation is the sorry condition of my radiator. I took the cover off to clean out the fins, and after clearing out a bird's nest or two, I found that about 10% of the fins would disintegrate into a pink powder when I rubbed my thumb across them. So I'm not getting 100% cooling capacity.
Even so, I'm going to see if I can run with what I've got. First step is to dump the existing coolant (which has gotten pretty watered down after repeatedly refilling it with a hose) and fill the system with premixed antifreeze. This may bump up the boiling point enough to keep everything from gushing out when the pressure releases.
Thanks again to all those who contributed. I'm going to answer your points individually in separate letters.
Marty