Well, partial success. I got the extractor bolts out using a pin punch and hex key without too much trouble. Drive side crank came off easily enough with a crank extractor tool but the left-hand crank was a complete #$@%* – it was locked solid, the thread stripped and it took three hacksaw blades, two drill bits, a cold chisel, lump hammer and an entire afternoon to get it off.
I guess that’s the price you pay for bottom brackets that last four years…
Next time, I think I’ll take it to a shop