More Carb Tuning Questions

Have you looked to ensure that the throttle blades are fully closed against the bores? If they are, I wonder if there was a casting flaw in the base plate where the idle speed screw is.
12.5 AFR at idle? That is too rich but I'm sure you know that. I recall the Holley instructions stating the "baseline" setting for the idle mixture screws is 1.5 turns out from their seats. Turning the screws in will lean it out, too much will cause a lean stumble when you tip in the throttle.