The magnet for the speedo can loose its magnetism. What did happen to me was my odometer was not working but speedo was correct. Took the cluster to the local reputable speedometer shop, they fixed the odo, but somehow "lost" the magnetism for the speedo needle and they were not able to re-magnetize it. Result: new aftermarket gauge mechanism...
Seems to be your issue, as explained in the other thread you quote at first. It seems difficult to re-magnetize the speedo magnet.
