And as Airbubba raised a lot earlier in the thread, the small vertical error margin does make altimeter setting something to return to - the difference between 29.92 and BHM QNH 29.97 just happens to be 50ft low - "with wheels hanging 30ft below that".
Well no, it really does not. You're confusing two different concepts. The obstacle clearance given by the PAPI has nothing to do with the obstacle clearance provided by being at MDA. And altimeter errors would have no effect on the clearance either way of the PAPI clearance.
The place where the airplane first started contacting trees was much lower than 50 feet below MDA and much lower than 50 feet below the PAPI so the approximately 50 ft altimeter error from not setting QNH on descent couldn't explain the accident.