Quick update. Started there engine this morning, and tried the three ECUs. Each reacted the same when ignition key turned ON - no error codes. When starting the engine, the engine RPM started at approximately 1000 RPM, dropped to approximately 850 RPM, then started to recover to a fast idle around 1300 RPM. The RPM decreased and settled at approximately 1020 RPM, and was quite consistent with very little change, up/down maybe 20 RPM each way. happy with the idle at this time.
My thoughts on the starting and subsequent RPM is that when the ECU does a systems check, as long as there isn't an open circuit and the appropriate voltage is present all is well - may be more to it, the ECU will not generate an error code. This check only needs the voltage to be within spec limits of the resistance.
When the engine is started, the voltage the ECU sees is dependent on the resistance of the component and thinking that this is a different part of the ECU programming If so, could be a reason for the engine start profile as I am seeing it.
Read on the CX500 forum that a fellow cleaned/replaced the PB connectors on a turbo, same FI system but has a boost function. It was mentioned that this also helped clear up the low RPM engine stumble that was being experienced.
In order to prove this, and knowing that after some 37 years, a little bit of crud/corrosion on connector terminals can be an issue, will start with the PB sensor connectors, take apart and clean - don't think new connectors will be required, but if so have waterproof connectors on hand. The first set of connectors to be addressed will be those directly related to an ECU input starting with the PB sensors. Can't hurt.