I too, like Mathew, wondered how a new Credit Card requirement fits with Airbnb's policy. You are correct on assuming that getting remedy for scratches, a stain, etc from Airbnb may be as much fun as getting root canal; in all likelihood they will chuck that to expected 'wear and tear'. The snuck-in extra guest is another matter; perhaps that can be prevented in other ways, such as making it explicit not to do so, as you do now (stated in caps). Personably, that guest may be a fluke event, your gentle place just doesn't seem to be a candidate for the type of guests that tend to be 'shifty'.