On my naturalist view, the fact that makes (a) but not (b) normatively justifying would be some fact about how the goal we're discussing at the moment is saving human lives, not saving mosquito lives.

But the question then is what goal you should have. It is easy to naturalise norms inasmuch as they are hypothetical and indexed to whatever you happen to be doing. (if you want to play chess, you should move the bishop diagonally) The issue is how to naturalise categorical ends,the goals you should have and the rules you should be following irrespective of what you are doing.

That's a natural fact. So are the facts about how the English language works and how two English speakers are using their terms.

Such facts aren't supernatural. OTOH, they fall on the analytical/apriori side of the fence, rather than the empirical side, and that is an iimportant distinction.