I wouldn’t want to keep @dandangerous holding his breath, so here’s the latest report from the ER:
Pre-op soaking in lye (because why not, just in case)
I don’t have a breaker bar of any kind, so the crazy answer was…TAPE! 
And then the even crazier move (PSA: do NOT do this to your Shapeoko, ever)
But the craziest thing is…it worked !
I can’t believe that whatever that black residue in the threads is, required so much brute force to remove the nut.
Now to clean-up the shaft thread, try on a new collet nut (luckily I had bought a few spares), and then…damage assessment, running the spindle slooowly.