>  the consensus currently is that the higher level classes like `Blackoil*` take cell indices, while the low-level `Pvt*` ones take region indices?

I was about to disagree in the context of saturation properties, but now I see that in the case of end-point scaling (or hysteresis) we *always* use *different* functions for each cell so the low-level calculators don't even need region indices, just saturation values.