Regarding your questions:
1. You cannot disable the clearing of the textbox.
2. Because the textbox is automacally cleared after the data from the postback is loaded, you cannot access the value entered by the user. Also there is no property that exposes the text entered by the user.
3. The text rendered on the image can be accessed through the RadCaptcha.CaptchaImage.Text
All the best,
the Telerik team