[golfbulldog]

I think you're right about the straight plane line being the key thing, as it is one of the 3 imperatives. There's nothing wrong with the concept of the flat inflexible plane, in regards to plane shift. Let's say you have a shift from the elbow plane to the turned shoulder plane on the backswing. You haven't bent the flat inflexible inclined plane; there are, in fact, two: the flat inflexible elbow plane, and the flat inflexible turned shoulder plane. You simply moved from one to another. This is my understanding of the concept.

That is the way I see it too.