Toggle Apple Mail "Classic Layout" with shortcut?
Here's the applescript for it. Set this up as an app or something but this should get you started.

tell application "Mail" to activate
tell application "System Events"
    keystroke "," using command down
end tell

tell application "System Events"
    tell process "Mail"
        click button 5 of tool bar 1 of window 1
        delay 0.1
        click checkbox "Use classic layout" of group 1 of group 1 of window "Viewing"
    end tell
end tell