First of all, I'd like to apologize for the late reply.
I investigated both issues you describe and indeed, even though PreviewSelected is handled, the SelectedItem holds a B object. We will further investigate the problem and provide solution for it as soon as possible. For that, I have added 1000 Telerik points to your account.
As for the second issue you describe, unfortunately I was not able to reproduce it. I bound a TreeView to a collection of objects. As soon as I removed the SelectedItem of the TreeView from the collection, the SelectedItem was null. I have attached a sample project that demonstrates this functionality. Have a look at it and let me know if I am missing something.
the Telerik team