We are introducing default font properties in the next official release, Q3 2010, which is due in mid-November. As of now, you can change them as described in this forum post, which you might have already seen.
The Underline property is ReadOnly, as there are various line types you can choose to underline something. You should use the UnderlineDecoration property in your case. All in one, this is how you set the FontWeight and the UnderlineDecoration:
span.FontWeight = FontWeights.Bold;
span.UnderlineDecoration = UnderlineTypes.Line;
As for the selection event, there is a SelectedLayoutBoxChanged event, which RadRichTextBox exposes. Have you tried using that? You might also want to check out RadRichTextBoxRibbonUI if you still haven't.
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