This is a great investigation but I have two small nits:

> the ZNB field is not empty and not garbage: it contains a well-formed 71-byte DER ECDSA signature, correctly Ascii85-encoded, with the right prefix and a plausible length. But it fails the cryptographic check instantly, because it was signed with somebody else's key.

Seems doubtful! I expect the forgers used a real signature from another card instead, so it has the right key but the wrong data. Reverse engineering the process as the author did and making up their own key wouldn't be of any value to the forgers.

> I built a little demo to check the signatures across California, New York, and Virginia: take a picture of the barcode and check it here.

This is not wrong, but should come with a little warning. A real verifier needs to additionally check the encoded data matches the human-readable data on the front of the card.