Microsoft reveals very few details about the OS itself. Other than it being a trade-secret, it would be rather self-defeating since this evolves fast on mobile platforms. You should only focus on the api that's provided, this ensures that you'll get store certification and hope that your app still works on later phone editions.
WinRT is that api, along with a smattering but short list of native api functions. You'll indeed get the CoreCLR derived from Silverlight, a trimmed version of the desktop CLR. It includes the language projection that makes it easy to use WinRT from a managed app.
P/Invoke is not supported. It is not a problem in practice, other than portability, Phone8 supports developing native C++ libraries that expose their classes through WinRT. Usable from any language that you can use in a WinRT environment, like C#, VB.NET, C++ and Javascript. You'd start such a library by using the C++, Windows Phone, Windows Phone Runtime Component project template. You must otherwise observe the same api restrictions.