Ultimately, what the pressure is at zero flow or excessive flow is irrelevant. What matters is the pressure across the coffee while brewing.

I'm not quite sure.

- The pressure at zero flow (blind filter) is always maximal, wether there is scale or not in the machine.
- The pressure at excessive flow (no PF) should be atmospheric if there is no scale (or not much). If there is a lot of scale, it can be higher. In the second case, one needs to grind coarser to allow a normal flow during the extraction. In the first case, during extraction, all the pressure is due to the puck and one needs to grind normally that is to say fine.

The pressure at excessive flow (no PF) should be atmospheric if there is no scale (or not much).

This is not correct.

You need to have an understanding of fluid mechanics and realize that the intersection of the pump capacity curve and the system resistance curve will dictate the pressure developed within the system. Yes, this is complicated a bit by the pressure regulating valve built into the pump but the rotary pump on the overwhelming majority of espresso machines has a capacity that FAR exceeds espresso requirements.