Art, this is starting to feel like one of those odd-ball puzzlers you used to post for us to diagnose.

Like you, and after doing my usual best to grasp at straws, based on the progressively worsening park position and other details posted, I'm having great difficulty imagining anything other than mechanical slippage at one end or the other on the shaft going through the firewall. The shaft itself is almost certainly one piece of metal. Given that the wiping area on the glass isn't moving (just the park position) then the short linkage arm on the end of the shaft simply has to be losing orientation with the plastic parking cam at the other end.

If it's not slipping in the slot under the 10mm nut then the plastic cam gear must be capable of slipping, such as under extreme loads like when the blades are stuck in ice or trying to move a load of snow. That might help explain things if the park position occasionally changes in steps rather than in continuously small increments from wear and tear. Anything else that could cause excessive loading between the wiper arm knurled studs and the cam gear?

If it's only an occaasional change then I'd just re-align the cam gear, tighten the 10mm nut over a serrated lock washer and scribe alignment marks everywhere to help spot any change the next time the park position moves.

All this discussion is bringing back memories of the many 140/240 wiper problems I had to sort out over the years, from contact corrosion on the back side of the cover plate to remounting dislodged magnets in horridly rusty motor cases, plus the usual relays, wiper arm slippage, switch stalks, pitted contacts and the linkage assembly under the dash. The 700/900 wiper systems are seemingly far less trouble prone.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now