3/8 NPT of course tapers, so the major diameter the thread is going to depend on where you measure it. The taper is .75" per foot, so if the thread is an inch long it would be .6125" or so at the tip, which is getting close to the mystery thread measurement.
This talk of the P6 wiring diagram being complicated makes me laugh. It fits on a single sheet of paper for crying out loud. Our 2012 Passat wiring diagram is a 263 page book (actually it's 630 pages, but the rest are about where all the electrical hardware is located around the car, not the...
All I can say is I've seen a flat ring used, and it failed quickly. The quad seal rings are designed for use in dynamic (i.e. moving) sealing. They are not designed for static use. I'd also point out that the DeDion tube uses quad seals, so Rover was quite aware and chose to use a round section...
I'd cut just the head off so you have plenty of screw to get a grip on, remove the runners and trim so you can get at the nut plate and then the usual story of penetrating oil & cursing to get the screw out of the nut plate. Good luck.