Alright, so I went back to the scene of the crime to survey the damage in the clear light of day. A few things became obvious to me:
- The driveside and non-driveside cranks, while cosmetically similar, weren't matched. This has nothing to do with the story but it surprised me that it took me so long to notice. Oops.
- The end of the bolt shaft, while buried about 1/2 cm down in the spindle had a ragged enough end that I could wedge the end of a needle nose pliers against it and spin the BB to back it out. Sweet!
- Despite what I had thought, I cleaned out the driveside extractor threads and found that they weren't bad at all. I can't say for sure, but they seem quite serviceable.
So forget all of the above! I just need to swing by the LBS and pick up a 170mm NDS mountain crank. That's gonna be like $5 from the used bin. Oh, and I guess I need a new bolt too. Meanwhile it gave me a nice excuse to drill a drain hole in the BB shell and clean out the shell threads. Whoever assembled it last didn't use grease in the threads, I think.