Well, it has been almost exactly two years since I started this thread with the last round of aggressive cleaning due to near failure of the latch and lock. The mechanism has been getting worse the last few months and the last fillup, I almost couldn't get the cap open at all.
Today I took out the latch and it was cleaner than before, no black stuff (which I now think was the graphite), but lots of white powder again (see photo). I hadn't added ANY type of additional lube the last two years. I used a Dremel with wire brush to remove it from all surfaces. This time, I decided to try a different lubricant, a thin amount of molybdenum disulfide grease, it is the only other thing I have left to try. I applied not only to the wear surfaces, but pretty much all over.
The lock mechanism was just as bad, but I wasn't in the mood to remove it and deal with the hundreds of parts falling everywhere, so I shot some spray graphite into it for now. Since the lock is still not really clean, it is still a bit stiff (and doesn't spring back), but a lot better than before.