Little_Dragon, you have done a great job of explaining this. I would like to add one thing. Changing the end point also changes the way the head bends at the neck. For a small rotation this should not matter much, but fot a large rotation it will become obvious (whether the new bend is worse or better than old one is another question).

In the image above the heads were twisted 64 degrees and bent 20 degrees.

I have been trying to find a way to overcome this problem, but with only limited sucess so far.