After much tinkering, attempting to accurately infer the a/c state by just the climate sensors I am ready to admit defeat.
I had accuracy to >95% – but as I discovered, that was only for certain situations. By that I mean that it was quite accurate at predicting if the a/c was on, in a climate where if often needed to be on. But not so accurate on cooler days where the interior might be naturally cooler.
Oh well. The flip side is that the switchbot button has proven more reliable than anticipated. The node-red flow just observes the exit code of the shell call and keeps trying until it works.