@Adisma, I think I found it
I deleted the C2 IAP object,
inserted the Cranberry's PhoneGap IAP object,
added some IAP events (I do not know if this has any influence on the outcome),
exported to Cordova,
made a XDK Crosswalk for Android build (to my knowledge with no special settings),
uploaded the new APK (arm)
and now Google says:
Your app doesn't have any in-app products yet.
Add your first in-app product
So I guess I can now continue my journey to get IAP working...
(right now I do not have time to test)
I still wonder where it went wrong with the official C2 IAP object.
(I prefer using "official" plugins.)