For what i remember, using admob plugin was not very hard, you struggle a bit but get it working... however, i understand your frustration with the "free crap", they make a huge advertise about their services but nothing work without some struggling(sometimes a lot), and the ones who knows how to do the things just hide away and won't share the knewledge, today for example i spent more than 6 hours just trying to make the google play game services work in my apk and it still won't work(at least i learned a lot about google services).
But to your question: The code to edit... well, hard to explain, but i can say that the best solution is to write a cordova plugin for it, android API for admob must be hard for your if you aren't a developer. Since you are using construct2 and want to make an APK, the best solution is to use cordova, and to use admob in your apk you could develop a plugin(as i said it may be hard) or you can find one, maybe the one from cranberrygame? or you can use the construct2 plugin that just use the cranberrygame cordova plugin.
The thing is, construct2 games are web applications, they need a native app(on android, ios, windows, etc.) to run on, cordova already give it to you, you just need to develop a plugin to it, in this case the plugin already exist, just learn how to use it. I'm just talking about cordova because intel xdk and cocoon all relies upon cordova to build the final apk.