Processing has a REPL mode which allows you to see live code (sketch changes as code is changed). Maybe that will be a good mode for you to explore. related to debugging, are you aware of the debugging capabilities of P3? It has the features that you mentioned except “to scrub backwards in time”. With respect to the latter, I haven’t seen any debugger doing that. As a matter of fact, one small issue with that approach is that after you leave a scope, your data is marked for gc and I don’t think you are able to step back in your code to restore those references. Mayb you have a novel approach. All ideas are good ideas, or at least, good to discuss. If you want more feedback, you can discuss them further here and different P3 goers will provide feedback if that is what you seek.
Lastly, I believe the Android mode is still looking for tools to perform debugging in the PDE, in case you are interested in exploring different concepts or tools.
Kf