It can debug, yes, but capability ranges from godlike for algorithmic issues to mediocre for subtle UI things. We are pretty close to your described app singularity if the app is within GPT’s wheelhouse.

Yes, you can iterate on the app and ChatGPT Work will then create new versions of the app.

If something in the UI of the app looks weird, you simply take a screenshot and ask ChatGPT to fix it. Or what I also like to do is add a logging functionality to the app. If something goes wrong, I then just upload the log to ChatGPT so that it can fix it.