Ok, so for those keeping score, here's the final...

I put in a total of about:
30 gallons of bleach (20 1.5 gallon bottles)
8 pounds of CalHypo powder (to "Spot Treat" some areas in the fillets of the steps and lounge)
5 pounds of Borax
25 pounds of DE
I also:
Replaced two DE grids
Replaced my original Rainbow 320 chlorinator with a brand new one

...before things finally went "static."

Once things balanced, I also found need to raise CYA again (which I did with three pounds of Dichlor shock)
Floc'd and Vac'd.
Backwashed a total of three times.
Opened and Cleaned the filter after the final backwash. The smell was indescribable!

For the last two weeks, my pool has been around this mark:

FC 3.5 to 5ppm
CC (none)
CYA 30-50 ppm
TA 120
CH 320

... but I think the pool ultimately won the war. I think I'm going to have to replaster. But that's a subject for another thread.

Thank you ALL for your help! I couldn't have gotten past it without ya'll.