Keyboard short cut to edit a cell in Numbers

System Prefences > Keyboard > Keyboard Shortcuts > Enable "Full Keyboard Access: All Controls", then you can use Tab to move between elements until your file list is highlighted, and use the arrows to select your file, in fact you should immediately be able to type a letter or use the keys as the default focus should be on the file list.

It's annoying that there isn't a simple keyboard shortcut for this. I suggest providing Mail feedback to Apple if you'd like to see it work properly.

For a workaround, try the following AppleScript:

tell application "Mail"
    set the selected messages of message viewer 1 to {} -- deselect messages
    tell application "System Events"
        key code 115 -- scroll to top
    end tell
end tell

You can make this into a Service and assign a keyboard shortcut to it from the Keyboard section of System Preferences.