Update:
I have managed to remove the EBB via a brute force method. This has left a groove of about 1.5 mm where the EBB was forced past the in-situ bolt. The damage is so slight it isn’t an issue.
I now have the EBB out, but the bolt is now still through the frame. The threads on the bolt are clearly stuffed, but I think the frame is OK. The middle of the bolt threads are gone, that’s why it spins, but I can’t get it to ‘bite’ on the lower threads. There is about 1.5mm of thread showing inside the BB shell.
I’ve tried a variety of levers to lever ‘up’ on the bottom of the bolt in the BB shell whilst trying to unscrew, but have had a spectacular lack of success.
I have never encountered a problem like this before, and I’m stumped.