Hmmm... Well, so much for that theory. I would expect the car to perform/feel different at 7,000ft in elevation than at sea level.
The transmission shift firmness is reasonably well calibrated to the engine output. Hence it all feels well and good when driving around town. Take the car to elevation and the engine produces much less power at the same RPM. Yet the trans shifts about the same. Different feel - it's as if the engine/transmission tuning has been tweaked.
This may or may not explain what you experienced.
I don't think your usage will have overheated or stressed the transmission in any way. Heck, it pretty much qualifies as normal usage. Ever seen the way Europeans drive their cars? Ever seen the roads in the Alps or Pyrenees?
All my older Benzs tended to shift more softly as the transmission got hotter. My '87 wagon shifts like butter after a good highway run in our 102F Texas heat.
If you're really concerned, have the transmission serviced and use a fully synthetic ATF. I have Mobil 1 in my wagon. Mostly I notice how smoothly it shifts right out of the garage on cold winter mornings. On the other side of the equation, it's almost impossible to hurt synthetic ATF through overheating - it's much more robust at high temperatures than old dinosaurs.