You are making this a lot harder than it needs to be. Placing two MOSFETs in series is on the difficult side. You can simply crowbar the power supply when thermal runaway is detected. If you use a PTC it will probably reset when the power is removed for a short time. The potential divider is OK but what will you do if your load fails open?
Detecting the detached thermistor is going to be difficult if it is measuring within the allowable range, maybe that is why you could not find an example.
Saving the last temperatures is OK. Heaters have a thermal resistance and inertia, you need to take that into consideration when checking for a change.