The radiator fan switch sounds entirely plausible to me. Same for a bad water pump. They have a finite lifetime, the impeller blades can break apart or wear down over time, limiting coolant flow. So if the radiator fan is working properly, low flow could also cause overheating because the water pump speed is usually tied to engine speed. That’s why at highway speeds it may not cause a problem, because the increased pump speed can overcome its worn out impeller, but when slowing down to idle, the pump may not be able to push enough water around. I saw some bizarre symptoms from a worn out pump in a '93 Mercury Sable not long before I got rid of it. The temperature gauge would read low and then shoot up if I floored it, along with not providing good cabin heat at low speeds either.