If you’re expecting this shop to fix the problem I fear you’re not learning from your experiences.
If you’re expecting this shop to fix it at no additional charge, I fear you’re altering reality.
And any mechanic that believes that he/she has properly verified/disproven an overheating problem by idleing the engine must have played hookie a lot in school… an engine creates a lot more heat when driving than it does at idle. It is possible to find a problem at idle, but it is not possible to diagnose the absence of a problem at idle.
Diagnosing overheating problems is not rocket science. Any competent shop can diagnose and correct the problem… but it will cost you more than $100. They’ll probably test the system for leaks under pressure, check the coolant for the existence of hydrocarbons, perform a “pressure leakdown test” if they suspect a headgasket leak, test the radiator for flow and “map” the radiator with an infrared thermometer for evidence of blockages, check the pump for flow, test or replace the thermostat, test or replace the radiator cap, and maybe even check the radiator hoses for evidence of cracks or failure (hoses’ inner liners can close up). If your Focus uses the radiator to cool both the engine and the tranny fluids, they may even check for evidence of passage between the two sections of the radiator.
Somewhere in all of this testing the real source of the problem will pop up. But it will cost you more than $100, so be prepared. Unfortunately, your only other option is to let the current shop continue to do nothing meaningful.