Poor idle and off-idle performance after distributor swap

OOPs, I guess I shouldda asked.
But it looks like you are on the right track.

A for the short shaft, somebody may have put a washer or a bearing, inside the D under the weight-carrier, and or not punched the bushing down far enough. You might want to take it apart and see what can be done in there. After the shaft comes down, you will need to put some shims above the plastic pinned collar, to take up the slack, so it stays down.

If nothing else, you can shim the I-shaft up.

But I'm still confused; Did you not say that the oldsetup was doing the same thing?
But yes 5* of rotor travel, traceable directly to the I-shaft slot clearance, or the bronze gear on the cam, is a lot.
In my readings, the bronze gear requires it's own I-shaft, different from the regular I-shaft; IDK, as I have never run into that, just remember reading something to that effect.