Ok so since the last update, the fan _does_ come on when the engine starts overheating when I'm at a stop for a minute. However, sadly, the temperature slowly still went up with the fan on. Turning on the heat brought it back way down--and, of course, so did turning the car off. So that said, I don't see how it can be the fan anymore. The reservoir still fills to the top, and after a long drive, after the car's been driven a while, the coolant level drops down to the top of the metal grill in the radiator.

I'm starting to wonder...given that it is only happening after some long-distance driving when I come to a stop, and that the heater can turn down the heat fine at a stop, but the radiator fan can't, can it not be the radiator itself? I'm at a loss, otherwise.

 

That, water pump or simply unbled air in the coolant. How old is the radiator? Did you have the coolant checked for combustion gases (indicating a blown head gasket or cracked head)?