Right, I got bored waiting so I plugged it back in; exact same behaviour. In the meantime, I had received an email from Asus support, saying "Please try resetting the keyboard by holding the Fn key + Escape for 15 seconds." This did a bit of a light show and hey presto, it's back in business. Screws in, keycaps back on, RGB set to 255, 255, 255, sorted! The child in question is not yet off the hook, as it's still cost me hours of my life.