I'm actually going to recommend you try something completely different. The Play Games team just released a new set of management tools that provides a web interface for you to interact with the games management REST APIs, including being able to reset your achievements. You can find them here, along with some instructions about how to use them.
Please note that as part of the setup process, you're going to need to go to the Play Developer Console, jump into your game, create a linked web app, and then use that linked web app's Client ID as the Client ID you enter into the management tools.