Great story, Blue, and I'm sure you and Hew won't mind me pointing out that your answers were for low level flight, where barometric error is dominant. If you are holding a constant indicated altitude, you are flying on a pressure level, and the height of the pressure level amsl is determined by a combination of msl pressure and the thickness of the layer of air below you. This, in turn, is determined by the air mass temperature. At height it is still true that if you have port drift in the N hemisphere your true altitude is increasing, but you don't know whether it is caused by baro error or temperature error or both.
Agreed, at FL300 obstacle clearance doesn' matter much!
Dick W