Can I use WPF's Full-screen UI on Xamarin?
No. Xamarin.Android and Xamarin.iOS expose the UI of the relevant platforms - you can't just use WPF on them.
However, if you separate out your "business logic" from the UI logic in your application, you may well be able to use the same business logic project, and just create new UI projects (one per platform). Of course, that relies on your "business logic" only requiring functionality which is available on Xamarin; more generally you'd want to create a Portable Class Library with that code in.
No this is not possible. You have to use the Native API's for User Interface on both IOS and Android when you use Xamarin.
You can definitely share a lot of code, but WPF will not work.
No. Xamarin relies on each platform's native UI stack. You cannot take a WPF UI (or a Winforms or Silverlight UI) and run it on iOS or Android. What you CAN do is take the underlying core of your WPF project (domain, services, data, business logic) and use Xamarin to create an iOS and/or Android native UI on top of it.