With no desire to say anything insulting, RC, and having read carefully what you write, I would say that you have a very *mushy* and non-substantial argument against slavery. Ultimately, it boils down to what you feel about it. But I also recognize that there are some ideas (they seem fuzzy to me though) that connect with your feelings.
The issue here seems simple:
If one relies on 'naturalism' (the way nature is, the 'laws of nature' if you want, and the mechanical world of biology within natural systems) it is not possible to construct an absolute argument against slavery. Slavery (defined as using another being for one's advantage with little or no concern for the welfare of that being) is in fact part-and-parcel of the natural world. This seems to me an irrefutable fact.
What you seem to be describing is a democratic reaction against the abhorred condition of slavery. What you fail to take into account is that that abhorrence that you feel, and which I also feel, is not shared by many people who now enslave others. IC notes that slavery worldwide is not diminishing -- it is increasing! There are therefore many people out there who are not on-board with your sense of things.
You do say, and I am sure that you do believe, that "Nothing is just wrong. There is no intrinsic or inherent right or wrong". If you did not believe that, you would not, and you could not, write that sentence. It contains a specific and non-qualifiable meaning. You either say it, and believe it, or you do not say it.
If there really is no intrinsic right and wrong, then there really are no right and wrong. Or, those are decisions of convenience made in a given moment. If they are 'principles' they are mutable principles. They might apply now and today, here, but not tomorrow and there.
The curious thing is that, and this seems obvious, in order to define an absolute value, it requires a law or a rule outside of the system in which the rule or law is applied. Clearly IC's argument moves inexorably to a divine mandate. Similarly, or relatedly, one might try to build an argument by reference to metaphysical principles -- but many here do not recognize those and in fact deny that such *exist*. Yet metaphysical principles are not part-and-parcel of the natural world, they are ideas and principles that arise, uniquely in us, and have a domain in some other space. (I view metaphysical ideas as being 'intrusions' into, and often against, the rules that apply in Nature).
So it seems to me that you really and truly are in a position in which you cannot build a universal and absolute argument against enslavement of another human. You can though state your preference against it.
I believe that I have seen and restated your position correctly and fairly. Though I have a strong sense that you will accuse me of being underhanded in some way?