I managed to reproduce the issue with the ensuring cell's visibility. It will be addressed in one of the next releases. In the meantime, I cannot give your work around due to complexity of the issue. The second behavior is expected because of AutoSizeDropDownToBestFit mode that you are using. Alternatively, you can disable the mode and specify the drop down width manually:
.radMultiColumnComboBox1.MultiColumnComboBoxElement.DropDownWidth = 500;
Notice that the version that I am using is the latest Q3 2011 SP1 (2011.3 11.1219).
I updated your Telerik points.
the Telerik team
of RadControls for WinForms is available for download
(see what's new