Holley Idle mixture screws non responsive?

A .484" has a pretty late intake closing point and with 9.0:1 CR it sounds marginal to me to get any decent driveability until you wind it up. If you can cram in as much timing as you want and it only improves, that may be a good indicator of low cylinder pressure, and in my opinion that's the beginning of your problem. Advancing the cam is a good way to get back lost cylinder pressure, increase manifold vacuum and also help ease your tuning efforts. 10-12 inches of mercury is pretty normal for big cams and having it drop in gear is also normal. The power valve you have seems about right.

If the timing is too retarded at idle it's possible the throttle blades are open too much to keep the engine running and that tends to pull more fuel out of the transfer slot, which bypasses the mixture screws. The proper position for the throttle blade in the bore is to have the bottom third or so of the transfer slot exposed to manifold vacuum and the ported vacuum port not exposed to manifold vacuum. Use that as a baseline to tune from. Also check the secondaries and make sure you are not idling excessively off those. Set the secondary stop screw just so the throttle blades don't stick in the bore.