I think that the difference between the clubshaft line and the imaginary straight line between PP#3 and the sweetspot is comparatively very small, and I am happy to reword my question to fit your requirement "that the sweetspot plane should be constant through the impact zone". The only point that I am attempting to make is that a golfer should continuously trace a SPL through the impact zone - and that includes the post-impact period - and that the resultant hand arc shape will depend on the angle of the inclined plane.
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