I think you and your instructor make some real good points Gibblet.
and there is no way in the world the shooter would ever know what happened unless someone showed them a video.
I agree. And the follow through in the example, I believe is when the arm is coming straight back as an opposing force to the arrow path. When the forearm/hand starts to move upward, it is not part of the follow through. Also notice that the arrow is well off the bow when the arm starts moving up. I think that there is a lot of relaxing happening in his draw arm/shoulder.
its impossible to drop your bow arm in anticipation. its impossible to do anything with your bow hand because it was completely relaxed to start with.
The bow arm in this example, I believe is perfect. What happens sometimes though sometimes in anticipation, is that the arm starts to drop before the arrow cleared the bow.
Very different for the longbow bow arm however, especially since there is no stabilizing device or much riser material. Your bow arm basically becomes your stabiizer.