I'm not sure what the temperature limit of the plastic coated 10k Grin thermistor is. I might not use it inside the motor can, however, you could use it on the outside. Bury it in the bottom of a cooling fin, on the back side so it sees as little wind as possible, and cover it with a dab of silicone gasket maker to secure it and shield more wind. The silicone gasket material is pretty easy to pull off also. The current settings in your CA for temp protection assume the thermistor is against the motor windings so use a 160/170C soft/hard limit. With the sensor on the outside of the unit, you'll want to lower these limits, I would say 70/80C should be pretty conservative- this would safely allow the windings to be twice the temp as the casing. Sorry for the inconvenience, put the 10k on the outside and adjust the temp setpoints.
If you want to ship back the motor, I'll pay and install a new sensor against the motor windings. Tangent gearbox disassembly instructions- these show a gen2 unit, obviously if there's a separate 3220 engine, you'd remove the end cap with the 6 allen screws instead of the snapring, but otherwise instructions are the same.
1. Remove the snapring behind the freewheel using a flat blade screwdriver.
2. Remove the M6x14mm screw in the center of the freewheel (this holds the FW against the GBX output). Thread in a long M6 screw, I've begun shipping these with the kits, an M6x50mm should do it. These M6 threads are in the GBX output piece; the long M6 will thread far into the unit and push against the motor shaft. Without the snapring in place, the entire gearbox assembly will pull out of the housing. Don't drop it, don't get it dirty. Once the output cover and large bearing are free, the assembly should pull out easily by hand.
3. Remove the motor key so the rotor will pass through the seal.
4. Remove the motor cover snapring same as the FW snapring.
5. Remove the motor cover (it's easiest to push the rotor shaft towards the windings a bit, it'll push the cover out), push the rotor shaft towards the windings and pull out the rotor. Make note of the spacers and wave washers on the shaft and their arrangement, don't lose any when the rotor pops free of the stator (magnets).
6. Epoxy the thermistor to the motor windings. Route the wires through the windings so as the thermistor can be glued securely to the windings. Ensure the wires are not interfering with the rotor air gap, ensure no epoxy is left in the air gap. Use about any high temp, thermally conductive/electrically insulative epoxy such as JB Weld (no kidding, works great). Most epoxies are electrically insulators due to the much higher content of plastic than metal additives, but read the label to make sure.
7. Reinstall in reverse order. Careful with the insertion of the rotor, don't let it slam into place.