Maybe u can try to use Cocos2d-Xna, because it using C#. I think, it is a better way to use
C# in Windows Phone for compatibility purpose, all the sample code are write in C#. In Microsoft website, u can download the IAP source code sample(in C#) and try to integrate to your game(I haven’t try yet). U can integrate leaderboard also by using OpenXlive(in C# again).
Cocos2d-x is a cross platform framework, but if u want to use IAP and Leaderboard for WP and Android, it is very painful.
I suggest or request Cocos2d-x.org or volunteers to help us to develop the IAP and Leaderboard binding for WP, iOS and Android.