When it says "Use the camera to take a picture of another screen", I stupidly chose the wrong app to do that. I selected an ordinary camera app rather than a QR scanning app and set that "always." So now I can't scan any QR codes. Is there any way at this point to change the scanning app that is used?
That is in fact what the developer, Martin, intended. The Mekorama app simply uses your default mobile camera to "see" the QR Code and then Mekorama decodes it and adds it to your library. As far as switching what apps are launched by other apps, you can google that information particular to your own mobile OS.