No mod is "100% safe", currently you can spam "instant win" and "complete all challenges" and it seems to be okay but no one know what they will check on ban waves, since they can check old logs and such, so I would use it carefully and not finish all stages too fast, fail here and there, and things like that to simulate a slower process.
To make in game purchases with Google play, you'll need to use an Unsigned APK in a rooted and patched device or emulator, here is an option if you don't want to root your device: