@Aurorjag For whatever reason, it seems like the iOS app doesn't currently permit using Google Play credit for game purchases; that's only an option in the Android version of the Stadia app. Please use the feedback option in the app to let the developers know that they need to fix this. Hopefully they'll get that capability added on iOS (and the Chrome web app!) soon.
@YoHeisenberg You can, and a few have been doing that. However, considering Bluestacks is not an official Android emulator, I would proceed with extra caution. There's no guarantee with regards to your account security.