...when I open the the Edit widget, the zoom-in/zoom-out buttons in the nav widget are greyed out...
You can see the same behavior in the Edit widget sample at http://resources.arcgis.com/en/help/flex-viewer/live/index.html?config=apps/config-edit.xml
All the mouse and keyboard navigation is still available, for example:
- Press Shift and click and drag to draw a rectangle around an area and zoom in.
- To zoom out, press Alt and Shift when drawing the rectangle.
- see https://developers.arcgis.com/flex/api-reference/com/esri/ags/Map.html for more.