I finished re-abusing the mule. Few notes:
- Removed any rust or tarnish from the previous treatment with a Scotch Brite pad
- Immersed the entire blade in saturated salt water through Wednesday
- Based on some notes I saw on corrosion testing, I then took the blade out, wrapped it in a paper towel, and put it back in the water with the blade sticking out, such that only the tang was in the water
- Removed it today
Interestingly, there was no further corrosion on the blade itself. The damage from before is still there, but no new rust spots appeared. Some spotting did appear on the tang, especially around the cutouts. The tang had not been immersed in water on the first pass.
So, it does look that whatever was corroding on the blade itself was isolated to that one spot along the edge, and once gone, the rest of the blade really is extremely rust resistant. I guess this further supports the theory that the corrosion is due to impurities in the metal, and not the LC200N itself.
One thing I noticed with my tests is that all my rust spots appeared near where the metal had been worked - the edge and the cutouts in the tang. It's possible that the metal had been burnt while being ground down and drilled, which may have affected it's corrosion resistance. However, that wouldn't explain the corrosion the OP saw.
Next step will be to reprofile the mule, to fix the edge damage, and see if it still functions as it should.