Well I was bringing this up in the context of normative ethics and it isn't at all clear to me what normative ethics would even mean if moral realism is false. My best guess is that normative ethics just becomes descriptive ethics (where we're trying to codify the ethics that in fact humans (or the West, or you) hold jointly. And I think everything in the OP holds true for descriptive ethics as well (except maybe instead of how people say people should act is replaced where possible by data about how they actually act). For non-cognitivist theories, as komponisto indicated above, intuitions are even more central- again we're just not really doing normative ethics anymore.

So yeah, my position doesn't quite work as stated if moral realism is false. But if moral realism is false then normative ethics doesn't quite work as stated.