We've checked and you are correct - you are unable to use Nullable DateTime with the new functionality. Thank you for reporting this problem. The fix will be included in our next internal build. I've updated your Telerik Points
as a small sign of appreciation for your efforts.
As for the second question, you apply two groupings simultaneously - one for Month and one for weeks. The other option is to implement a custom group description. I'm sending you a simple project demonstrating how you could use the second approach.
Feel free to contact us in case you have any problems or concerns.