Something the conventional story about ethics gets right, with which you seem to disagree, is that ethics is a society-level affair. That is, to justify an action as ethically correct is implicitly to claim that a rational inquiry by society would deem the action acceptable.

You're just redefining "ethics" as what I called "social ethics", and ignoring the other levels. That's treating ethics as a platonic ideal rather than as a product of evolution.

Of course, you've given good evolutionary-biological reasons why most of the time moral judgments do motivate, but, I claim, there is no necessary connection.

In the view I'm presenting here, judgements by a person's personal ethics do always motivate action, by definition. Moral judgements computed using society's ethics don't directly motivate; the motivation is mediated through the person's motivation to accept society's ethics.