in reply to
Is this a simple, robust, and maintainable design?
I'd also appreciate some suggestions for improving the system.
I would suggest reversing the logic of variable names starting with 'NOT_'. It can be quite confusing to follow the logic once the whole system becomes pretty complex. So, why not name your states like 'CHECKED', 'VERIFIED', etc. and functions like 'is_checked', 'is_verified', etc. It's much easier to follow the reasoning, IMO. Notice that you don't have variables like 'NOT_BROKEN' or 'NOT_COMPLETE'.