W/r/t the question of scrubbing backwards in time, I suppose I was thinking that the state of the program would be recorded manually by the mode itself. So restoring a previous state would mean looking up that information in a data structure maintained by the mode rather than the JRE. Of course, there would be some memory limitations to how much history you could store.
I may have confused the issue by calling it a debugger, because what I had in mind wasn’t to reimplement what Processing’s debug mode already does well.
Anyway, thanks for the feedback — this is good food for thought