[Mathew]

The plane of the #3 accumulator is formed on two of it's four sides by
1) the left arm- defined by the straight line between the hand and the shoulder

Yes. You use the angle of the left arm and from this you can keep drawing a 90 degree 3 more times to get the four corners of the plane. Only when you have this relation with the accumulator plane can you then define a truely vertical plane (Jens plane) to relate to both acc#3 plane and the inclined plane. When both these planes directly face each other by the left hand turning towards the inclined plane, they directly intersect each other on a straight line. Because they directly face each other they are parallel just in the same way the edges of the cover of a book are parallel when you open it up. This intersecting line in which the secondary lever assembly demands that wristcock be at exactly 90 degrees.

and assuming a flat left wrist

2)the clubshaft- defined loosely by that straight line from the hands to the clubhead

The clubshaft is very close to being onplane when it is turned however due to any hookface position needed, this might not actually be exactly the case. The sweetspot or the longitudinal center of gravity always stays in the accumulator no.3 plane and it is this that we reference....

We're using a zero shift plane in this example and looking at say the movement of the #3 accumulator plane from impact fix to the top in relation to the swing plane.

Since the left shoulder is not on the swing-plane (always "in front" of the swing plane- from impact fix to the top- assuming a #3 accumulator) and the hands are always on plane- then the #3 accumulator plane is closed to the swing plane and not parallel to the swing plane at any point in the backswing.

So your comment "parallel to" throws me. What do you mean by "parallel to" and where and how does that happen?

Planes are always referenced as though they were the horizontal. Above the plane is a far more accurate description. The left arm does go into the inclined plane at an angle. And because accumulator no.3 plane is a left arm plane so does it....

They are parallel in the context if you have yet another plane (I'll have to name this one too) on the tangential line of the left arm angle vertically through accumulator 3 plane, the intersecting lines of the inclined plane and acc no.3 plane will be on a parallel angle when moved in a parallel direction away from the tangential line along the line of Jens plane.

The plane shift variations question is out of context.