Hey, Art. Thanks for stopping by! I really appreciate it, and your level-headed meticulous thinking is something I don't entirely have nailed down. Plus, I'm not that familiar with the ECU. Most problems are things people have encountered before. This one seems like a real nut to crack.

I think that checking things at the ECU is the way to go - that verifies entire circuits at once. On that note - is it Pin 8 on the ECU that is the input voltage from the AMM? I see there are 3 pins into the ECU from the AMM.
Is "It is 8 which tells the ICU what it needs to know about ignition advance from what the ECU learns from your AMM about engine load." referring to pin 8 on the ECU?

And yes, I ran across ballast resistors in different systems trying to search RWD for ballast resistors. The vacuum does drop off, though the throttle plate is still open the same amount. I can see if I can feel the IAC move/click when it cuts out.

So many things to verify, but at least it's a finite list now!