Happily, either the effects of PB Blaster or driving a little with the car (carefully) took their toll. The remains of the castle nut came off within ten minutes with just a little tapping with my new chisel. The threads seemed to be in good repair, but I chased them with a new die (only $4 for the 12 mm, 1.25 mm pitch one at NAPA) to be safe.
The ball joint separated so easily probably a good blow with a small ball peen hammer would have been sufficient. I suppose the taper was really lubed up, or else the driving around loosened everything in, I confess, a seriously risky way.
The boot was a bit dried out and beat up from the brake cleaner. I applied PB Blaster, for now, since it's said to restore rubber. Probably have to replace it soon after the rough treatment it saw in the last couple of days.
I note I also priced new lower ball joints at NAPA today. These were part of my backup plan.They had two types, one with a lifetime warranty, one without. One $27, one $52, IIRC.
Forward to full control arm removal, then bushing replacement. More update on this in another thread.