start_proximity helped. Now I can see the controls when proximity sensor is covered.

However, is it normal, "sudo lfocus start" never returns until quit with CTRL+C?

If one keeps camera in the stand by, not closing the lens cover or camera application, but just leaving it to the backgroung; then using keyboard to type a message for example will adjust the focus and turn LEDs on/off? Maybe you could make a change, that if Camera application is not in the foreground, then do nothing although the keys are pressed.