1. If QFE = QFF then the measured pressure at the aerodrome is the same as the actual sea level pressure (near as it can be calculated). If the pressures are the same and the location is the same then the aerodrome must be at sea level.
2. QNH pressure is the sea level pressure calculated back from the observed QFE using ISA lapse rate and the known aerodrome elevation above msl.
3. That's going to come out to the same as QFE as we have already established that the airfield is at sea level, there is no elevation above msl and therefore no difference between QFE and QNH.
So what if the atmosphere was other than ISA? The first statement still stands, it doesn't depend on temperature or lapse rate. The last statement still stands as there is still no elevation above msl. Neither are dependant on ISA lapse rates.