Go to the coolant sensor and disconnect the sensor and ground the wire on the connector to see if the gauge goes the other way. You may just have a bad sensor.
If nothing happens you will have to trace the coolant wire to see if there is an open in the circuit.
You did good work but like I said with today's electronics in today's cars even the best mechanics may have to depend on the dealer if they don't have the proper scanner and software to do the job.
Let us know what the problem is when you find out.