<p>Lately I've been having problems with overhangs not adhering when curvature is outward (stringing across instead) that look like what you'd see at insufficient temperature, and that go away with temperature jacked up a bit (PLA at 220 °C, which is a bit extreme) or fan disabled. Is it possible that the hotend thermistor has drifted and is no longer accurate, making the actual temperature lower than nominal? Or do these things fail hard when they fail?</p>

It is indeed possible that the thermistor is broken (yet not sure). I am aware of two types of issues with thermistors:
The contact (soldering) is broken, usually due to the temperature extreme variations. The thermistor will indicate the maximum temperature in case of PTC or minimum temperature in case of NTC. In some cases, due to vibrations or other factor, the contact will eventually touch, showing improbable jumps of temperature.
The thermistor is broken internally. In that case it will just indicate a wrong temperature. I can't say if constant temperature or constant delta.
For how to troubleshot the component, however, EE.SE seems more convenient.