Two comments. First, your point about counterfactuals is very valid. Hofstadter wrote an essay about how we tend to automatically only consider certain counterfactuals, when an infinite variety are theoretically possible. There are many ways that the world might be changed so that Joe one-boxes. A crack in the earth might open and swallow one box, allowing Joe to take only the other. Someone might have offered Joe a billion dollars to take one box. Joe might aim to take two but suffer a neurological spasm which caused him to grasp only one box and then leave. And so on. Counterfactuals are a weak and uncertain tool.

My second point is with regard to determinism. What if the word in general, and Joe in particular, is nondeterministic? What if QM is true but the MWI is not, or some other form of nondeterminism prevails? Ideally, you should not base your analysis on the assumption of determinism.