I was going to remove it entirely and just carry it loose, or adjust the clip so it wasn't so tight (still not decided on that). I prefer to remove clips to adjust them (they don't always bend nicely when screwed down), so it was going to come off either way.
I finally got it. First I cut a large flat slot with a grinding disc so I could use a large flathead. This usually works, but the heads were still giving away.
I gave it a few freezing and heating cycles to try to loosen it up. I don't know if that really did it or my twisting efforts were just finally enough.
It looks like there is some kind of clear glue under the clip and on the threads, vs. red or blue thread lock.
The mystery glue seems excessive, and it also seems like these should really be either torx screws all around, or maybe just philips for the clip screws. Hex heads never work out.
The good news is I finally got my non-black Nobleman. I planned on buffing it down so it would be a mirror finish like the smaller model, but it's been so long since I saw a nice brush job this good I am going to leave it alone.