And yet, there are many situations in which an observer does in fact look at two groups of people and claim that group A is better off than group B. On your view, are all such observers unjustified in all such claims, or are some of them sometimes justified? (And, if the latter, is there any reason we can't affect the world so as to create such a situation, wherein we are justified in claiming that people are better off after our intervention than they were before?)

Well, there's the anthropological concept of the psychic unity of humankind — we may have different values, but our ways of thinking (including our values) are not wholly alien from one another, but have a lot in common.

And there are also things we can say about human values that descend from cultural evolution: we would not expect, for instance, that any culture would exist that did not value its own replication into the next generation. So we would expect that people would want to teach their ideas to their children (or converts), merely because societies that don't do that would tend to die out and we wouldn't get to observe them.