Well, the 850T that would shift into any gear at any time turns out to be a "key" on the side of the shifter that is moved up and down when the selector "trigger" on the shifter is squeezed. The key finds the notches for P, N, D etc. The key piece is gone, probably broken by a gorilla when the microswitch failed. By the way, the switch is a 15 minute job. Real simple, the only hard part was pulling the dang green wire free of the harness plug so the new one could be inserted.
Now to figure out what piece needs to be replaced, or if a whole shifter needs to be transplanted.
This 850 stuff is not so hard with 30 years of RWD experience as groundwork. Kind of fun actually. Thanks for the advice so far and the nifty Volvospeed pages.