My RT-PCR test was done 70 hours before my scheduled departure to Thailand (via Doha). Does anyone know what happens if there is a flight delay?  Are the rules documented somewhere so that I can show them to the check-in agent at the Airport?  Thanks! 

might be problem with checking in, that would depend on airline flexibility.

You might be forced to do pcr at the airport.

There might be chance for the pcr at doha airside before boarding bangkok leg

They should go by the schedule departure time instead of the delayed departure time. If the flight was canceled for a day or so that would be different.