You could draw that one picture in a position - but just tell me how your going to get there. If the right and left shoulder are on a plane any line between that point is going to be onplane too. So if you use a point between the shoulders center for simplicity - that will be onplane too(not quite true but close enough for our purposes).
Now if the stationary point stays onplane - the right shoulder stays onplane - how can the left shoulder leave that plane. Now try to visualise the shoulder motions and ask yourself, how can you have a top of the backstroke where this could happen....it can't....
Independent movement of the shoulders? They are not a single T-bar. In fact this independent movement need not even occur during the downswin. If both shoulders are forward at the top then they curve forward from the spine. I agree any point between the shoulders would need to be on plane, just not that the spine would need to be between the shoulders - in a linear fashion that is!
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