How do I change mouse settings instantaneously from Terminal?
AppleScript is a nice way to go ! 

tell application "System Preferences"
    reveal pane "com.apple.preference.mouse"
end tell
tell application "System Events" to tell process "System Preferences"
    tell checkbox 1 of window 1
        click
    end tell
end tell


It works on Mavericks. It might need some modifications on older OS X because the UI of the mouse preferences changed.