I'm not real crazy about the idea of the saintly sufferer myself. But it is the fundamental founding myth of Christianity, and I suppose if the person who's doing it is God himself, or 1/3 of God, than the objections you raise won't apply.

And Free will is the fundamental founding myth of ethics, so arguing against it is not going to help your case for Utilitarianism. If you don't have choice, then there is no point in arguing over what we ought to do. You are just going to do what you do, and that's the end of it. There are people who argue otherwise, but that requires a form of free will which is compatible with Determinism, sort of. That is the basis of Dan Dennett's Compatibilism, best expressed in his "Freedom Evolves."