Thanks for that.... You just saved me a few hours additional research on Pearl to find out whether I'd got it wrong (and misapplied the calculus for interventions that are impossible in the original system)!

Incidentally, I'm quite a fan of Pearl's work, and think there should be ways to adjust the calculus to allow reasonable backtracking counterfactuals as well as forward-tracking ones (i.e. ways to find a minimal intervention further back in the graph, one which then makes the antecedent come out true..) But that's probably worth a separate post, and I'm not ready for it yet.