Finally an atheist proper, with views and questions

[Lucian]

So for me the strong form of error theory is what I currently hold to whereby a) moral talk makes claims about mind-indendent normative properties, b) no such properties exist, therefore c) all moral claims are in error.

This has always been a fairly simple one for me, but perhaps I'm being naive.

If one views morals as the necessary rules of interaction between members of a society then premise b fails and the argument collapses. If morals exist within a society rather than within a single mind then you simply don't have a problem.

It's a pretty simple thought experiment to show that any society that doesn't have a proscription against murder is going to be very short-lived. The same can be done for any behavior that is a net negative for the society. Any society in which might makes right is unstable, because even the mightiest must sleep lightly with one eye open. This also explains why the proscription against murder is very specific: Thou shalt not kill a member of our society who behaves properly. In-group murder is prohibited, while out-group murder is either encouraged or at least not nearly as heavily discouraged. Even more interestingly, members of a group that threaten the integrity of the group typically find themselves targetted more viciously than any outsider (apostates, heretics, and Republicans who don't back Trump come to mind).

And I'm not just talking about the Bible, although it is a treasure trove of lovely examples. From our drone strikes to the child labor that makes our food and clothing, our lofty morality seems to only apply to our little clan.

I agree with you that this is a reason that morals tend to have survived and been useful in societies. I think this might sit under the category of evolutionary debunking arguments, although I am poorly read on that at the moment. That said, I don't think it collapses premise b. Moral error theory is about whether or not mind-independent normative reasons exist. Normativity related to some intrinsic motivational force of these properties, or something that has a "ought" built into it. Your examples about society are good descriptively - but then if morality is just that set of facts then why add the concept of normativity at all
One question about possible social origins of morality would be how far such arguments get us. For example, we could claim that morality has these benefits, but it doesn't prove that therefore there is no mind-independent property. Something I have more reading to do on