The best way to track down your short is by disconnecting items, like the heater, the thermostat, the switchover device and isolating different components while plugging the cord back in. A simple process of elimination. Your element may be heating ok but could be shorting out thru the casing, did they check for that? I would reinstall it and plug the fridge in with it connected and disconnected, see what trips the breaker. Another thing, if you are running off a GFI they can be ultra sensitive, might take less than a direct short to ground to trip the breaker.