[Bigwill]

But what about plane shifts - they are common and well-defined within TGM orthodoxy. The plane angle varies but they always keep the same staright plane line.

Isn't the constancy of the same straight plane line the key thing ?

I am keen to understand this properly so please if anyone else can help - I would be most grateful. The point about an "inflexible" inclined plane is OK for zero shift patterns but needs to be further defined for those that shift planes - surely?

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.