I’m responding to your last section first as everything else will get built around it:
Your mind and body are linked together. You cannot separate the one from the other. This is a section of the quote that JJ posted above from the Ethics of Liberty:
You cannot transfer control of your body to anyone else. This is why any such contract stating that you have transferred control is invalid. The transfer cannot ever occur. Enforcing a slave contract would require aggression, using violence to take something that is not yours. This is why slave contracts are not compatible with the NAP. Obviously, slavery can be legal in any society, but it cannot be legal in a society based upon the NAP.
Well consent is not enough. A person can rescind consent at any time, because the will (control over mind and body) is inalienable. Legally, it is theoretically possible to sell yourself into slavery in a society that permits slavery, but even in such a society, you have still not actually sold your will (control over mind and body). For the slave owner to get you to do his bidding, you must always do it yourself, as you are the only one in control of your will.
So it would require aggression to enforce the contract, as the slave owner would have to use violence in order to take something that is not his own.
Let’s suppose you agree to be whipped should you break the contract. Okay, but the contract right from the start is considered invalid as you never can transfer your will (control over mind and body) to another person. Even if you broke your agreement, you would not have taken anything from the other person. So you have not committed a crime. But if the slave owner then whips you for breaking the agreement, he would have committed aggression because he has used violence on you when you have not taken anything from him.
It all stems to the inalienability of the will. Since the will, control over mind and body, cannot be transferred, any contract stating that is has been is nonsense. If someone were to enforce a slave contract, it would require using aggression. It is aggression because the will was never actually transferred. It still remains yours, no matter what the contract states. That is why slave contracts are incompatible with the NAP. Indentured servitude is another story, but I don’t think we are talking about that anymore.