Thank you for your feedback regarding this matter. There is a feature request added for this functionality. Feel free to add your vote for it here: http://www.telerik.com/support/pits.aspx#/details/Issue=10738
In regards to the check box this is also a known limitation and a case for it also exists: http://www.telerik.com/support/pits.aspx#/details/Issue=12254
Inheriting a control does not mean that a bug will be introduced. Especially in cases like this one. If you have any doubts regarding some functionality, you can always open a support ticket with your control and ask for advice.
I agree that the check box issue is quite unpleasant. Unfortunately the issue is deep in our framework and addressing it is not an easy task. In regards to the read only state for a drop down list, such feature differs from the control semantic. However, due to the interest shown in this thread, a feature request is created.
Feel free to add your votes for those cases since this increases their priority among the other tasks.
I hope that you find this reply useful.
All the best,
the Telerik team