Incorrect diagnostics, terrible customer service

I bought a second hand ford Everest from an independent dealership which experienced overheating after 3 weeks. I immediately informed the dealership who told me to take it to get a quote so that they can pay to repair it. I chose to take it to Ford directly (as one should be able to trust the OEM). The performed their diagnostics and advised me that only the water bottle should be replaced and quoted accordingly. The dealership approved and paid for the repairs in good faith. I drove it for a few days before the overheating notifications resurfaced. I then took it back to Ford who then told me they don't find any fault. I then took the car to another RMI approve repair shop where they almost immediately identified a blown head gasket and the root cause being a faulty thermostat. Since the dealership already paid for the initial recommeded repairs as advised by ford, and 8 months have passed they had no obligations to further assist with payment toward the repairs done by the second RMI approved shop. I reached out to ford again to assist since they miss diagnosed the overheating event and they declined my request. Now i had to pay R13000 out of my pocket because ford secunda couldn't find the Fault. How is it that a ford workshop is unwilling to help but another repair shop could find the problem after looking at the car. During the whole process the manager was difficult/unwilling to communicate or assist in solving the problem. This mistake cost me R13000.