Actually think you're all overcomplicating this. If the performance is good, most people won't notice a perfect shadow. Getting the head turn to feel right matters way more than the physics of a fake light source. Obsessing over tiny details like shoulder shadow direction just kills the shot's energy.
Yeah that shadow thing is such a dead giveaway. I read a breakdown that said you have to treat the main light as a fixed point in the scene, not just on the face. @caseyhart is right about syncing to the keyframes, because if the light position in the room is static, the shadow direction should stay the same on the neck and shoulders even when the head turns.