What is meant by 'offensive' use? Is the use of a nuke to defend against an invasion offensive?

@skibidist apologies for not figuring that out sooner. Lacking any great suggestions, I'm going with: outside the recognized borders of the country using it, and not a test.

How is this list exhaustive?

<edit> I'm dumb, did not see the "other countries" option

The intent here is that this list is exhaustive; if there is an offensive nuclear detonation, at least one of the categories can resolve positively. That is: "country" and "non-state actor" include all possible actors; between that and "unattributed", something can resolve positively.

Did I miss anything there? Any edge cases worth worrying about?

Extraterrestrial Aliens.

OK, yep, that's an edge case I missed. Hopefully not worth worrying about, though. I think that could result in a situation where an alien state offensively detonates a nuke and nothing here resolves in response.