Yes, I agree, there has to be some kind of a circuit from the cranking battery to the starter motor which it is somehow unable to access. How can I identify this circuit and troubleshoot it?

Re the pyro battery disconnect, it hasn't discharged, so the positive lead is very much still connected to the battery. I am aware of this feature, as it is also implemented in Range Rovers from when BMW owned Land Rover. But sadly, that's not the issue.