All your points are certainly correct, obviously as you deal with this stuff for a living
However, I have the SMS 122 and it is common practice to calibrate with only the pH 7.01 solution. I belive it even stats that in the directions, as long as you are using it in freshwater of mid pH ranges.
See another thread here
Reading through that I wonder 2 things. First, can the slope change over time, as in a few years? Second, the dial on the front of my SMS122, the part you turn to select you pH, has always been a few points off. I assumed it was just a poorly squashed on knob but know I wonder?
Also, the SMS122 has a +/- .2 accuracy, which can vary the CO2 levels a bit when discussing CO2 levels.
Fish stress is not necessarily an indication of high CO2, it can also mean low O2. Remember the CO2 does not displace O2 in water so technically you could have 100% saturation of both, or only one. In my opinion the best way to describe setting CO2 levels based on fish stress levels would be..."Increase Co2 levels until you observe fish stress and intense pearling at the same time, then back off on the CO2 rate until fish are fine" By intense pearling I don't mean a few bubbles rising from a cut leaf or a little on the bottom of some rotala leaves but lots of bubbles everywhere
This woudl insure that fish stress was due to CO2 and not low O2.