I did configure AdMob for my game today, and it seems that the very basic assumption for AdMob in UE4 is wrong.
In project settings you can specify only one Ad Unit ID, this means that we can use a single unit per application. However an ad unit is not an account identifier, it is, just like the name says, just an add unit.
IMO the Ad Unit ID should be moved to the BP node that is displaying the ad banner (or interstatial in the future). This way we can use different units for a single app and the whole system gets flexibility needed for future extensions.
And if this kind of functionality is already in the works, as it seems impossible that the current solution is the desired state, it would be really great to hear something about it already