its not a 5 minute job to change. i would very much doubt he would have changed it in the time it takes for a football match to run
running lean enough to overheat the engine..? i doubt it. he'd have got a code:25 if that was the case..
this one is a real bummer. let's review your troubleshooting again;
- swapped the coolant pump: no change
- swapped the thermo/ran without one: no change
- flushed the rad: no change
- redid the head, valves, gasket: no change
- tested the rad fan, it works: no change