Unsuccesfully Trying to Bind a Nested Property Model to a DataGrid

I wrote a minimal version of my problem, it only contains a DataGrid with 3 columns.

Requirements:

(a) The two ComboBox-filled columns must be implemented with identical definitions and processing. They must be essentially copy&paste from each other.

(b) I cannot use the simple approach, based on List<string>. I need a more featureful custom object.

Without further ado, there you go...

         Trying to Bind a Nested Property Model to a DataGrid 

TIA,

- Travis