The difference in tolerance is indeed very interesting. To my experience, bot paros and bettas are quite tolerant to suboptimal conditions, although coccina-group bettas seem to be more susceptive to stressy environments like badly furnished tanks, whereas paros seem less content with distubances like being moved. My coccina-group bettas (I have hendra and uberis) are the most relaxed fish I know, and tolerate everything I do in the tank without even swimming away. I could easily grab them with my hand.
Given enough hiding places, they don’t seem to care much about their environment. I haven’t noticed any difference in behaviour or colouration at different temperatures or pH-values.
I can’t really say why they were so unimpressed by the low temperature, but I find it interesting. I’m going to pull out the heater in some tanks here, to see what happens at 18 degrees.