The important thing to realize is that any “voluntary slavery” contract requires constant consent from the slave. This is the result of an inalienable mind and body and will, or whatever you want to call it. When a person agrees to be a slave and do his master’s bidding, he never really transfers control of his mind or body or will or whatever you want to call it. This is Rothbard’s point. There was no actual transfer. The master still requires the slave to consent to anything. And if the slave ever rescinds his consent, then the only way to force the slave (of course, some slaves may rebel until murdered) would be through aggression. Why is it aggression and not just plain old regular violence? Because there was no transfer of title to the slaves body.
As I said before, one might be able to argue that certain body parts can be transferred as they are alienable, but something seems quite off about this. But, even if we had a successful transfer of title regarding an eye or a kidney, the body as a whole cannot be transferred. Someone could transfer control of a kidney, and maybe eye transplants will be done (perhaps they already are, though I have not heard of any), but the mind/body as a whole cannot be transferred.
I undertand that you are trying to say that a slave owner believes he owns the will of his slave, but that is not Rothbard’s point. Rothbard is just pointing out that the control has never been transferred, and that is what slavery is about: who is rightfully in control of any given body. That rightful (just) control cannot be transferred [legal control can be transferred so long as the legal system allows for it].
I understand that this is rambling, but I really have no idea how to get this point across to you if you haven’t already gotten it from reading Rothbard. Rothbard’s theory is based on title-transfer. Without an actual transfer of title to the body (remember, the owner still doesn’t control it), the contract is invalid. Enforcing such a contract would require aggression. Thus is it incompatible with the NAP and libertarianism.