Simon,
The behavior that occurs when you press 'back' is consistent in the interface, still. That is to say, users can press the back button and select "ignore changes" at any point during form entry in order to cancel the form and return to the menu.
It has worked well on some projects to replace the final screen with a simple prompt that says something like
The form is over, please check the box below to confirm that it is complete
[ ] Confirm
and a conditional text below that says "You can press the Next button to submit the form"
Apologies for the introduced difference in form submission. In our testing we have found that although it takes a short time to re-adjust to the new "confirm" option that users have been able to recognize the significance of a "complete" form through a combination of the progress bar, green submit button, and the inability to swipe to "complete" without needing the landing page. If you are seeing significant troubles after users have had a chance to adjust, please feel free to reach out.
-Clayton