There was a solid clue during setup...when sliding the connectors into the harness...they are whisper-tight. Meaning: Not at all. Solid money is on worn out/loose harness connectors but we'll see if banging on ECU does anything after the normal "give up and die" timeframes of 20-40 mins.

RESULT: Loose harness connector(s). Now to figure out how to either tighten/tweak the harness connectors...or replace them 🫤

There is still a mystery (to me, at any rate) on why the engine would sometimes simply shut off like a switch and sometimes stutter-stop, and why sometimes it would re-start immediately and othertimes re-start 10-20 mins later. My hypothesis is that by simply getting in/out of the car while diagnosing I was re-wiggling the loose connector(s) such that they had good-enough connections, and the changing fault symptoms were a result of different pins or a different combo of pins losing contact.

The end result is the same: Fix all the connections and do pin-drag tests on all of them to make sure they are sufficiently grabby.

Thank you to everyone. I hope it was fun for you too.

Don't overlook that when you have the connector disconnected from the board that the solder joints are probably less-stressed. I didn't see the common cracked solder joint problem mentioned in the thread.

For some reason the internet is not picking up the old threads when I search. I only came across one. I've seen the others though.