Conclusion
I'll do my best to keep that article up to date.
If you ever encounter difficulties, always prefer to post in the How do I forum, with as much details (and possibly the capx for your project) as possible rather than simply leaving a comment to this tutorial.
Notice also this covers only the steps up to Alpha/Beta stage as I'm currently in the process of transitioning from such a stage to Production with my own application.
Nevertheless, having it working in Test mode/Alpha should be no different than Production.
If ever I encounter subtleties or major issues in the transition I'll update this tutorial.
Remember, before final publishing to change the "Test mode" properties for both the IAP and AdMob plugins in C2 !