Hi Liam,
The pressure reg normally will reach 4volts + under load this indicates pressure increase in the rail, the (P)ulse (W)idth (M)odified signal should show max off time (signal is broadest at top of waveform) which looks about right in the waveforms shown. Normally the IMV defaults to full pressure when disconnected so in it's max off state = max flow hence pressure buildup. Is there only an IMV on this model? some have a pressure regulator and IMV (dual point control) single point control (IMV only) will have a pressure relief valve fitted. (see my post fuel pressure blues where I had a problem with this.
Did you take the 1st and 2nd waveforms when the fault occurred? We need to see what the PWM is doing at max pressure (is it calling for high or low pressure) if it's calling to lower the pressure then that means it cant control hence probably the IMV. Sorry for all the questions, I would expect the IMV to go max on to try to reduce the pressure if it's getting to high.

Mark