I'll answer your questions in order.
1) Unfortunately, there is no way to expand the RadTreeList without a postback. Only the data you view on the screen is bound to the control. This is done for performance reasons. In order to expand the RadTreeList client-side, the whole tree would need to be transferred with each postback. For large trees this would be prohibitively slow.
2) This is fairly simple to do. It's called recursive selection and Telerik has a guide on how to implement it. Here's the link:
Hope this helps!