One more thought: I believe that contemporary Shimano cartridge BB's (UN72? UN73?) have "cups" (threaded sleeves) which are deeper than on-topic BB cups. It's possible that such a BB would find some healthy threads left, deeper in the shell. I'd still want to chase out the shell with English taps, though, to help the new BB thread in correctly, and to help it meet the remaining healthy threads correctly. The spindles are also available in a variety of widths, so you could run a triple, as you are wont to do.
Peter Bridge
DenCO