I’m not sure what that exception means but your best bet in tracking down the problem would be to do like I said before and set breakpoints throughout various parts of your code and step through using the debugger to see if you can catch the problem code.
For your second question, do you mean you want admob to notify you in lua when an ad as been shown or you want to call a lua method and check to see if an ad has been shown?
The former you would need to setup a LUA listener as described in this guide: http://docs.coronalabs.com/daily/coronacards/wp8/communication.html
The latter you can just add another C# listener like the other ones you setup in the tutorial and have it return true if admob showed an ad or false if not.