I've found the problem in my disk tool. The three-byte cluster record in the FIB block was being improperly constructed. This caused the program loader to get nonsense sectors, resulting in a crash.

I confirmed this with a known-good disk image. The files it contained were extracted, and a new image made with those files. That new image ran without problems.

I spent some more time doing cleanup and comments, and exercised the options a bit more to gain confidence in the results. The disk tool still needs some work though. For instance, data file handling is incomplete, handling disk-full conditions is iffy at best, and using the tool at the command line is awkward, with lots of easy-to-forget switches.