I am sorry for the delayed reply, we have been busy with the coming Q3 release.
Sure, I created a sample project based on some of your code and I have attached it here.
The column object is not a visual object that can be set as a drop target. It is enough to set the cells as drop targets (as you have done) and then add a handler for the drop events to some of the visual parents of the cells.
Hopefully this will help you,
the Telerik team