I don't see you addressing what I find to be the most important point, though I realize now that I didn't emphasize it enough. If we know that leaded gas was phased out at different rates because legislators mandated different schedules in different states (I say "if" — I don't know if that's true), then any hidden cause Z would have had to produce not only X (decline of lead at a certain rate) and Y (decline of crime at a similar rate) but also the legislation which caused X in the first place. Many plausible Z's become absurd if that is the case. To use your example, population density might conceivably determine both gas consumption and crime, but surely not the behavior of state legislatures implementing federal mandates. I find it hard to imagine history getting closer to a controlled experiment than that.

Does that mean that causation is proved? Of course not. But it does mean that the evidence here is stronger than garden-variety correlation.