Perhaps I have missed it, where is our initial ignition timing? Are throttle butterflies at idle so far open to cause you carb to be running in transition circuit instead of idle circuit. A simple test is to adjust idle mixture screws,,,,does this cause significant difference in idle speed? Crank initial timing up to say 15 to 20 degrees which will likely increase idle speed, then adjust speed back to normal using carb idle speed adjuster, with screw idle mixture screws out from all the way in, out say to 1.5 turns...Any improvement?

Have you tried earlier recommendation to bypass ignition wiring by jumping directly to coil from battery.

Your idle vacuum reading is extremely low unless some crazy camshaft. ,,are you certain of no major vacuum leaks,,,,power brake booster,,,intake to head, cracks in intake manifold. I've even run a garden hose on low pressure on a running engine and been surprised by what I have found.

Any bad cylinders,,,pull plug wires one at a time with engine idling best it can with vacuum gauge connected. Any cylinders with zero change in vacuum reading?

Carb float levels ok?