But if agent X will (deterministically) choose action a1, then when he asks what would happen “if” he takes alternative action a­2, he’s asking what would happen if something impossible happens.

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would happen if something impossible happens.

But since this is the decision process that produces the "happens", both "happens" are the same "happens". In other words, it reduces to:

asking if something impossible happens.

Which is correct. Because the CSA deterministically chooses the best option, checking each action to see if it is best is synonymous with checking it to see if it is possible.