With respect to your first question, yes, currently there is no way to prevent the user from selecting, say, ten files. The reasons are that neither Silverlight nor Flash provide API for the OpenFileDialog that would allow us to do such thing.
Regarding your second question, the correct event is FileValidationFailed. I suggest that you visit the following demo
for a reference.
With respect to the last question, currently there is no way to do that with RadAsyncUpload. We will research the possibilities for adding such functionality for future releases.
the Telerik team