So today I've reconnected the new CPS I installed and plugged the EZK box back in (I still have the wiring connector for it apart to access the terminals), and the car started up straight away. It's run for about 30 minutes without any problems idling. EZK was just sitting on top of the radiator support with wiring connected.
I have not touched anything else electrical in the engine bay other than unplugging the AMM and removing it to get room to remove cover over the clutch for fitting the flywheel look, the wiring to the CPS and getting access to the EZK box connector wiring to do the CPS function test. Can't take car on road yet as rego has run out. I have another (new oldstock bosch omg) AMM I want to test out but now the car is running so I'll put the EZK box wiring all back in place then try the AMM.
It's revs vary a bit as the o2 sensor output swings and the ECU responds, but I think that's normal for the LH 2.x setup? When the ECU drops the revs the motor sounds a bit rough, then they come up again. It's only varying between 50 and 100 rpm with the swings. My multimeter was showing the O2 output not swinging properly after about 15 minutes of idling but there was no CEL to indicate the ECU was unhappy with it.
So I still really do not know definitively what was making the car randomly stall when driving last year which was the reason I stopped using it and parked it up as all the symptoms and stuff I've looked at only appeart to provide circumstantial clues.
The discovery of the failing crank pulley seal on the oil pump assembly was a fortunate side-effect though so I'm glad about that and saved would could have been a more major failure had I never investigated the CPS aspect.
So it might have been the CPS (due to all the metal bits, oil, dirt etc inside the pulley from the failing seal), or it might have been the EZK box (perhaps it has got some bad solder joints), or the wiring for either (connections just got loose over the years). Or the AMM.
Hopefully the car passes a rego inspection flawlessly then at least it's usable on the road again.