Assuming the ground is entirely missing - open circuit, no, the 100mA blue light would outshine the two 2-watt panel lamps. But, according to a list of ground connections in the closest manual I have, the cluster wire shares a single-point ground with about a dozen other functions.

The headlight ground having high resistance would explain the blue light on low-beam, but the instrument panel ground being other than solid, is the only way the dimmer could shut off the errant blue light. It only takes 1 volt to get those warning lights to glow.

The easy first step would be to wiggle the cluster plug. Next would be to locate the ground point at the transmission tunnel.

As for the shade being broken, I don't know. It is a nice theory. I've had enough of these clusters apart to guess it isn't likely without some kind of really unusual damage, and I know Bob has had this car for a lot of miles.

Fun armchair diagnostics though. Aren't you itching to tear it apart and see what the real answer is?
--
Art Benstein near Baltimore

"Well, sir," the cowboy replied, "circle flies hang around ranches. They're called circle flies because they're almost always found circling around the back end of a horse..."