Shawn,
I just don’t think that posture sets the plane of the swing. I think the hands do for most players. The shoulders will react to this and help to maintain what the hands want. Posture is important to keep the integrity of the swing plane. As well as having a dynamic swing plane.
I agree you just can’t set the posture and have a good swing plane. The hands are important. The hands send the barrel into the torso/posture vector. It can be looked at as an geometry problem.