1) Construct a full-blown DAG of math and Platonic facts, an account of which mathematical facts make other mathematical facts true, so that we can compute mathematical counterfactuals.

If mathematical truths were drawn in a DAG graph, it's unclear how counterfactuals would work. Since math is consistent, then, by the principle of explosion, the inversion of any statement makes all statements true. The counterfactual graph would therefore be completely uninformative.

Or, perhaps, it would just generate another system of math. But then you have to know the inferential relationship between that new math and the rest of the world.