Its a TSP if you shift to that plane in Follow Through or just Stay on it throughout the swing.
Zero Shift would have you TSP throughout , zero shift is very uncommon however. Singe shift would have you staying on the TSP after Startdown, double shift would have you stay on the lower plane (Elbow Plane or whaterver you choose). You could add another shift and shift from the Elbow back up to the TSP Triple Shift ..........any and all combinations are available to the golfer, but shifting can be hazardous of course.
The lower planes through the ball have a pronounced feel for going "left" or IN , which they in fact do. The steeper planes have more a feel for the UP , which is geometrically true as well.
Follow Through......Both ARms straight, Impact Hands, Flying Wedges , club still on plane would be your ideal, yes. Plane Angle dependent upon personal preference.
This assumes we are talking about the common situation where the Shaft lays flat on the Inclined Plane as opposed to the Clubhead only describing a Plane of Motion. Homer did allude to this bit of business........which would require a change to 1-L-5. Homer was that kinda guy. Single Horizontal is non (shaft) planar for instance , the shaft describes a cone, the clubhead rides a flat horizontal plane though. You can forget this caveat if you want......too much coffee today. Im sorry Bear ........where were we.