After much screaming and tearing of hair I have finally gotten In-App billing and Play Services sign in to work in my Android application - kind of. Google should really be ashamed of themselves, the process has been gruelling to put it mildly. One example of extremely important information that is mentioned kind of like an afterthought on the tutorial pages: "You must create the new client ID in the Google Play Developer Console, not in the Google Developers Console."
I have now reached the point where my debug APK works with Game Services sign in (leaderboard and achievements), and my release APK works with In-App billing. But not both at the same time. I know that debug APK does not allow for In-App billing, but why does not the Game Service sign in work when debug sign in works? I have triple and quadruple checked that package name, client ID, application ID, debug AND release certificate SHA1-fingerprints and everything matches up perfectly, but still no luck. I get the dreaded "There is no linked app associated with this client ID." It's obviously linked, as it works using the debug certificate.
Are there yet another restrictions or minor detail I have to think about to make this ")=#(¤/"¤() work? :-)