After doing the above, when my son used the car again he said the speedo has been mostly working. Every now and then it stops but then starts up again. All I really did was jiggle a lot of wires, and unscrew and re-screw everything. That makes me believe it is just the ground screw, and all the other fixes out there work because by unscrewing it you've removed enough oxidation to make contact. I'm leaving well enough alone at this point.
And nobody has read this board since I posted anyway...