[comdpa]

If you shift during the Downstroke, which plane is the preselected plane? Or, is there only ever a preselected Downstroke Clubshaft Plane in a zero shift Downstroke?

Technically, using ANY plane, whether downstroke or backstroke, if one does not shift from one plane to another, then that means it is a "zero shift" stroke. A zero shift is not limited only to the downstroke.

If on the backstroke I elect to shift from an elbow plane to a turned shoulder plane then that is a shift.

If the Turned Shoulder Plane is the angle from the the plane line up to the rear shoulder at the end of the Backstroke(no matter what type of Shoulder Turn took place), how can the rear shoulder move downplane at start down on anything but the Turned Shoulder Plane?

Sure it can. I merely have to switch planes. In fact, you will see that such shifts are pretty common on Tour. Usually you will see a move from the turned shoulder plane to the elbow plane. But remember the words of Mr Kelly here. Any plane shift is hazardous.

I can't picture this comdpa Are there any photos or diagrams that could clear this up for me?

Another question. For how long should the rear shoulder move downplane? I see many down the line sequences showing the rear shoulder moving above plane before impact, but few with it remaining on plane to both arms straight. What is ideal?

As regards photos and diagrams, you may need to ask our resident imager Mr Annikan...

You may not like this answer, but the rear shoulder should move downplane as long as it needs to. As I observe in all my students, there is too much around as opposed to "down".