I'm going to go with the high-reading gauge for now. While there is still some risk in that, I think that the system pressure will at least be in the ballpark, if not a little low. I can adjust as necessary when the better gauge arrives. I also had a conversation with the local hydraulics shop. They said that they can test a gauge for a small fee. I'd have them do that today, but I don't have it with me.
The oval washer is a mystery that I may never be able to solve. I suspect that it's due to the valve being short a couple of bits. One of the missing parts is an o-ring that JD calls "packing". I'm not sure what the intended purpose is, but it would also serve to keep the valve plunger centered. It seems to me that allowing the plunger to wobble would cause both of the unusual wear problems that I found in the old valve. For now, I plan to check the replacement valve periodically for wear and address the issue if and when it comes up again.
While this may not seem like the best practice, time is not on my side. I return to school in a few weeks. My property grows into a jungle each year while I ignore it for 9 months. I have my summers to get the place in shape again. This year my wife wants to restore her Karmann Ghia in our garage, so I have that to clear out, in addition to the usual list.
Thanks again for the suggestions, info, and wisdom. I'll update with the final result when I'm confident that I have it worked out.
Cheers!
Goat