For iAP I was never able to get the offical iAP to work properly. I swapped to the Cranberrygame iAP plugin and was able to get that working pretty quickly.
As for AdMob, I have never tried the official C2 plugin. I use the Cranberrygame Charboost plugin for ads and its working well (at the moment anyway).
As for your black screen issue. Id just be systematic about testing it. You said you got a successful build recently. Remove everything you added since that build. Then just go one step at a time. Add in your Admob plugin, do a test build to see if it works. etc. Just add one thing at a time and test it. Hopefully that would allow you to identify what is causing problems.