I'm not seeing the Escape/Tab issue when I try in our demo. I set focus to the font drop-down and click Escape. Focus is back in the document. Then I press tab and it is consumed by the editor properly. If you are changing focus scopes then you could be causing this issue.
Anything that is a menu or toolbar should always be a focus scope. From the MSDN:
"A focus scope is a container element that keeps track of the FocusManager.FocusedElement within the its scope. By default, the Window class is a focus scope as are the Menu, ContextMenu, and ToolBar classes. An element which is a focus scope has IsFocusScope set to true."
So we have Ribbon as a focus scope since it more or less is a toolbar. This allows you to have focus set in the content area and for Ribbon to be able to have the commands in its button react to the current focused element in the content area.
But please note that you must keep focus within the content area or you could see some ribbon button disabling, for instance if your Window somehow gets focus and not the content inside of it.