Probably a mere verbal disagreement, but this strikes me as an attempt at a sociology of morality, rather than a metaethics. Suppose it were a historical fact that all new moral rules were introduced by prophets who got them from angels, and adopted because the prophets worked extraordinary miracles. That would settle the (proximate) question of why our society has the moral conventions it does, but wouldn’t settle the question of realism etc. (If it seems like the angels-and-prophets situation would settle things in favor of a theistic version of realism, consider that the prevailing moral rules in such a world could be just as contradictory as in ours.)
I’m pretty skeptical of cultural evolution models in general because they depend on an analogy that’s more vibes-based IME than formal, or that, if formalized, doesn’t incorporate certain highly relevant differences: the discreteness of organisms, genetype-phenotype distinction, non-intentionality of mutation, etc don’t really carry over such that I don’t really know how to cash out a claim like “the goal of a culture is self-propagation.”
To address some of your specific examples:
“Stealing” like “murder” is a trivial rather than substantively convergent example because the term already includes social disapproval. It’s substantive that there are rules in any society about when you can take stuff from someone. But having rules around matters of likely dispute is something you can account for bilaterally and intentionally.
Reciprocity can also be accounted for bilaterally and intentionally; there are convergent instrumental reasons to adopt a reputation for reciprocity. If an alien scientist in 100,000 BCE started progressively culling the hunter-gatherer bands with the highest rates of reciprocity (and secretly enough so as not to activate intentional mechanisms like “oh the gods are annihilating any group with overly strong reciprocity norms, we should adopt weaker ones”).
Xhosa prophecies: as you noted, people noticed that the prophecy was harmful and inaccurate, and adjusted their practices. The evolutionary model would involve differential success for societies adopting and not adopting prophecy. (Note that a less extreme example, potlach, has been able to persist for many generations across many societies.)
“Being celibate is good” has been embraced by multiple independent large-scale civilizations over large time scales; historical Christianity and Buddhism strongly claimed the moral superiority of renunciate over family life. Early Christianity grew quickly despite being even more anti-family. The basic survival of monastic institutions for thousands of years belies any very broad generalization from the failure of the Shakers. Differential reproduction rates might explain some of the tendencies towards pronatalist ideology over time, but I’d expect the effects to be slow and we see conscious state projects to instill pronatalist norms driven by concerns about military superiority, which itself is a dynamic that lends itself to selectionist explanations if you don’t think too hard but again I think you can get there with direct agentic/strategic intentions.
Possible synthesis: when you tell a joke and it offends or otherwise doesn’t land, you should update in the direction of playing it safer. Enough of this will put you in a situation where your best retrospective judgment is that you should (prospectively) not have told the joke.
However, there may be intermediate degrees of negative feedback where you’re updating in the direction of playing safer, just like you’re updating in the opposite direction when the jokes land, and on Pace’s account that would be an appropriate time to apologize even if you don’t (currently) regret the choice on net.