Joe: Look at what finally worked!
In Finder, hit command-shift-G, and enter "~/Library/Containers/com.apple.mail" (no quotes) in the field.
The folder that comes up in blue is the one you want to move to your desktop.
Restart your MBP immediately after moving that folder to your desktop...before reopening Mail.:dope slap: The content of the View menu changes according to the options that are currently selected. Deselect show column layout and Show bottom Preview should appear in place of Show Side Preview.
