There wouldn't need to be a two-second delay.

As soon as the lens cover is opened, the software says "was the cover previously opened in the last two seconds? If so, start the torch, if not, start the camera".

Your idea of starting the torch by opening the lens cover with the shutter button pressed is also a fine one, but there are other mods using the shutter button for other purposes (such as bringing up the task switcher) so this may cause problems. But there might be other ways to do it, such as opening the lens cover while covering the proximity sensor.

Regards,
Roger