Rich, these malfunctions always have a simple cause. It's finding that simple cause that makes us want to tear out our little remaining hair or take up golf. I would like to suggest that you try two things.
If turning the idle mixture screw has no effect on idle speed or quality the idle circuit may have an obstruction or blockage. Remove the idle mixture screw, spray in some carburetor cleaner, and then follow that up with a blast of air from your compressor. I have had tiny particles partially or completely plug the idle fuel circuit and the above procedure restored a good idle.
Also, it couldn't hurt to adjust the valves. That would rule out that issue. I would be especially concerned if they are too quiet. That may indicate they are not seating fully and are at risk of burning.
If the plugs are darker after idling in the driveway than after running faster on the road the float level setting may be too high or the needle and seat not adequately controlling fuel flow.
The possibilities are endless but it seems like setting everything that can be adjusted to recommended specifications is an essential starting point.
From there, swapping parts (distributor, carburetor, coil) from a properly running engine may help isolate the problem.
When you find the culprit you will say "How did I miss that?" That's what I say to myself every time.
