Providing a property for changing the text of the close button is in our ToDo list and we will implement it soon. As for changing the look of the close button, you can do this by providing custom skin for the RadToolTip. More information on how to do that is available in the documentation, section RadControls Prometheus Fundamentals / Controlling Visual Appearance
As for your last suggestion, try setting the Sticky property to true. When it is set, the RadToolTip will remain visible when the cursor is over it.
the Telerik team