If you plug in the AV-Cable Display is also switched off, indeed anything that has the shape of the plug will work.
Yes, that will save more power if your delay is very short. But if your delay is not very short, that set_backlight saves more (because the video output driver consumes current in the av plug trick).
Switching the display off using the display button (either by the script or before you run it) should save even more than either of the above (because sensor is also powered off), at least if delay is not very short. The problem with this solution is that manual focus cannot be used.
