Thank you for your questions.
Please refer to our online documentation
for the first question.
SelectedText is only valid while RadComboBox has the focus. It corresponds to the part of the text that is selected in the text box part of RadComboBox. Just like a stand alone text box, when it loses focus, all its text is deselected, therefore you can not get the SelectedText property by clicking on another control.
Please write again if you have other questions.
All the best,
the Telerik team