I don't have a good answer for the rest of your comment, but I can answer this:

Where I really trip up with this argument is in the 'granting moral status' step. What does it mean if I decide to say 'a fish has no moral status?'

Drescher does a good job of making sure that nothing depends on choice of terminology. In this case, "a fish has no moral status" cashes out to "I should not count a fish's disutility/pain/etc. against the optimality of actions I am considering."

You can take "should" to mean anything under Drescher's account, and, as long as you're consistent with its usage, it has non-absurd implications. Under common parlance, you can take "should" to mean "the action that I will choose" or "the action I regard as optimal". Then, you can see how this sense of the term applies:

"If I would regard it as optimal to kill weaker beings, then-counterfactually beings who are stronger than me would regard it as optimal to kill me, to the extent that their relation to me mirrors my relation to the weaker beings under consideration."

I didn't give a full exposition of how exactly you apply such reasoning to fish, but under this account, you would need to look at what is counterfactually entailed by your reasoning to cause pain to fish.