In order to have your RadCalendar instance display the month view based on a change of the selected date variable, you simply need to set the RadCalendar.FocusedDate property, server-side using RadCalender's PreRender event, to your selected date variable. In your case, you could use the first day of the month that is selected in your ComboBox.
I addressed a similar request in the Display required month on RadCalender
forum thread. The difference was that the selected date was set using a RadDatePicker. Code is provided, which was a modification of the Calendar/DayRender Event
Regarding your requirement to format the background coloring of the selected month, you can reference the above demo, which can be modified to color all days of the selected month. Also, refer to the Rad calendar multiview. Hide months views greater than RangeMaxDate
for some coding assistance for this.
Also, please note that there was a bug related to setting the FocusedDate property in your current version that was corrected in the newest Q1 2012 release. See the RadCalender section of the Release History page at http://www.telerik.com/products/aspnet-ajax/whats-new/release-history/q1-2012-beta-version-2012-1-201.aspx:
"Fixed: When set dynamically the FocusedDate property of RadCalendar with multi month view is not applied before the next PostBack"
Hope this helps!