Is it allowed by Windows Phone application certification rules, to only have users be able to return to a previous screen with the use of a hardware back button?
In my scenario, a user clicks on a setting button and lands at a settings page. He changes settings if he decides to, and all the changes get saved automatically, like in iOS.
But currently, the only way for the user to get out of the settings screen, is to press a hardware Back button on the phone.
Will such implementation of navigation functionality pass the Windows Phone certification?
As far as I understand the terms of the navigation guideline is your usage the right way to go.
See this cheat sheet for more informations on the design guidelines.