Replace the [on|off] Switch with an "Install" Button
The initial behaviour of the switch is not a boolean toggle operation, but one is shown the installation prompt instead, so replacing the switch with an Install button and then replacing the button with the switch once the extension is installed is the more intuitive way to go.
So on initial click of the "Install" button the extension is installed as usual and then the switch can be shown in place of the button for toggling on or off the extension.