I don’t think there are many pro-IP libertarians around any more to answer your main question.
Correct me if I’m wrong but, you seem to imply that utilitarianism can justify anything if it makes a certain number of people feel a sufficiently large, cardinal, collective ‘happiness measure’, but if you believe in ordinal utility, you should know that there isn’t really a happiness measure as such - you can’t add ordinal numbers together. The only useful number, for economists anyway, is the price paid for something in terms of demonstrated preference. Utilitarianism does not necessitate moral relativism, nor does it necessarily preclude moral absolutism, I don’t think. If you believe in ordinal utility, you should adhere to the non-aggression principle absolutely, because otherwise people will be demonstrating preferences which aren’t really their own. You can’t maximise utility if that’s the case. Is IP compatible with the non-aggression principle? I think there are arguments to show that it necessarily infringes others’ self-ownership in a way that acquisition of scarce, real property does not.
Utilitarianism is a form of consequentialist argument and ‘natural rights’ is predicated upon a deontological paradigm. In other words, utilitarianism says that we can only know what is right by what happens after we do it, and the ‘natural rights’ guy says, ah, but someone can possibly predict that a certain consequential pattern will repeat reliably in a set of instances relevent to a specific category of objects, i.e. categories of objects follow pre-existing rules natural or inherent to the situation - objects will behave in accordance with their nature. People are not all the same, of course, just like no two objects can be exactly the same and still be two objects, but they are a category, which implies sufficient common ground to infer the existence of a “human nature” common to all of them, and from this basis one can infer the existence of a priori rules, which must exist and should be derived, even if this cannot ever be done perfectly or entirely from our point of view.
Of course there is the concern that one will never know exactly what the rules are. One will never be able to derive them completely. Unfortunately, if you allow this concern to bother you, and you are consistent with it, that’s solipsism. You don’t really know that the objective universe with its inferred physical rules is really there at all, I mean, maybe someone in very high places is just having you on for a sick laugh, but that’s not a terribly useful state of mind to be in.
I agree with you that natural rights begs the question in the end: even if the rules do a priori exist, the reason we should pay attention to them must be to maximise utility, and due to the knowledge problem, the utilitarian framework is useful in terms of deriving what they most likely are, just like the empirical scientific method is good for determining what we know, or are forced to assume, must be the rational rules governing the physical universe. However, I also think that utilitarianism begs the question: how does one define and differentiate good and bad? Must it not be in terms of one’s nature, even if one does not or cannot fully know what that nature is? I mean, no one would deny that an applied scientific practice, like medicine, is concerned with the nature of the human body it contemplates, yet at the same time, one is going to judge their doctor’s efforts - and the efficacy of the medical practice in general - by their success in fixing whatever it is that was wrong. One will not think much of a theoretical physicist if his equations are consistently contradicted by empirical observation, no matter how right he thinks he is.
I don’t think there’s a choice between rationality and empiricism, or that it’s the case that one is completely wrong and one is completely right. They’re just different ways of thinking about things. Conjecture vs hindsight. An artifact of the way the human brain thinks about things. You need to use both, I think.
You know, it’s the same as the age old question… Do you worship God because you think He’s right based on your Earthly mortal experience, or do you know right from wrong actions because God tells you the difference? How can it be both? It just seems paradoxical because the human mind is like Janus - one face looking into the past with the clarity of hindsight and another looking into the future with the power of logical heuristics (or lack thereof).