I think this is right, except possibly for the part about no prior metaphysical meaning. The later explanation of that part didn't clarify it for me. Instead, I'll just indicate what prior meaning I find attached to the idea that "the virus replicated wrongly."

In biology, the idea that organs and behaviors and so on have functions is quite common and useful. The novice medical student can make many correct inferences about the heart by supposing that its function is to pump blood, for example. The idea preceded Darwin, but post-Darwin, we can give a proper naturalistic reduction for it. Roughly speaking, an organ's function is F iff in the ancestral environment, the organ's performance of F is what it was selected for. Various RNA features in a virus might have functions in this sense, and if so, that gives the meaning of saying that in a particular case, the viral reproduction mechanism failed to operate correctly.

That's not a moral norm. It's not even the kind of norm relating to an agent's interests, in my view. But it is a norm.

There was a pre-existing meaning of "biological function" before Darwin came around. So, a Darwinian definition of biological function was not a purely stipulative one. It succeeded only because it captured enough of the tentatively or firmly accepted notions about "biological function" to make reasonably good sense of all that.