Just like moving queen to E3 is instrumentallly-right when playing chess, but not morally right. The difference is that in the chess and heroin examples, a specific reference point is being explicitly plucked out of thought-space (Right::Chess; Right::Scoring my next hit,) which doesn't refer to me at all. Mugging an old woman may or may not be moral, but deciding that solely based on whether or not it helps me score heroin is a category error.

You seem to have intpreted that the wrong way round. The point was that there are different and incompatible notions of "right". Hence "the right theory of what is right to do" is not circular, so long as the two "rights" mean differnt things. Whcih they do (theorertical correctness and moral obligation, respectively).

I'm no good at math, but it's my understanding that there was an idea of what it would look like for someone to solve Fermat's Problem even before someone actually did so. I'm skeptical that 'solving metaethics' is similar in this respect.

No one knows what a good explanation looks like? But then why even bother with things like CEV, if we can't say what they are for?