I just had a nice talk with an employee at Admob about how to increase my ad revenue. And his recommendation was to add an interstitial when the players exit my game apps. I did not read through all the terms and conditions but I do believe that the Admob official knows the standard regulations and would not recommend this behavior if it was not allowed.
Technically I am going to implement it in this way:
1) Catch when the user clicks on my exit button or hits the back button on the main screen
2) Check if an interstitial is loaded and ready to be displayed
-> 3a) if it is not ready, i let the user quit without delay
-> 3b) if it is ready, I show the interstitial and listen to it being closed, when it is closed, I exit the app automatically