2013 Prius c overheating

Hi,in January it was -20 something and I was driving to work as usual when I noticed the temp hit 110 so I pulled over and let it cool down,no coolant.

I changed the waterpump and thermostat last year,and again after the overheating started,I also bought a new radiator cap just in case.

oil is clear and not milky,car runs great otherwise except cold air from the passenger vent,rest blow hot.
Radiator fan works.

A mechanic had my car for about 6 weeks,he told me he spent 4 hours on it @$140cnd/hour and cleaned my brakes,he wants $740cnd for the work.
On Friday I picked up the car and drove 115km/h temperature never rose above 91,so I thought it was fixed.
Monday morning 5am I leave for work,I drive about 6km doing 60-100km,almost as soon as I reached the highway temperature hit 110 so I pulled over and let it cool down,all coolant had gone.
Hose from engine to radiator cap was hot,the other side was cold.

the mechanic told me to remove the thermostat,but that doesn’t fix the problem and has me not wanting to fork out that $740.

anyone have any ideas?

I’ve tried everything that I can think of.

 

I had time today so I:
Done the balloon test on radiator,it filled slightly maybe 100-150ml coolant slowly as it reached 91 degrees.
Tested thermostat in boiling water.
Checked that radiator fan functions properly.
I jacked the front passenger side up and burped the coolant system.
Test drove the car for 20km at 120km/h,temp never went above 91 degrees,no coolant loss after it cooled down.
I plan on doing about 100km test drive on the highway on saturday to be sure.

Then I get to confront the mechanic,he used my spare water pump and it looks as if that's all that he done and I can do that myself in under 2hours,his wife test drove it which is included in his time.