At highway speeds the fan should have no effect on cooling since the "ram air" due to the vehicles speed is sufficient unless the airflow across/through the radiator is obstructed. So I wouldn't pursue that. I lived in Dallas for a while and it was not uncommon for the car to run at 105 C and hotter in traffic in the summer.
I'm not familier with diesels except in principle, but if a gas engine runs too lean it will run very hot; is this true also for a diesel???????