Airbnb should have paid you for the nights they stayed that how they did with me once,
In the future why not just charge for the unit and not per person? save getting stressed out,
My apartment sleeps two, if one person books the have a friend stay Im ok with it, the only thing I worry about is any more than two but in 4 years that has never happened.
Also I have some 2/3 bedroom villas I charge for the villa not per person,
Many times we have had a couple book a villa and had family or friends stay over, as long as the person who booked is aware they are liable for any damage done by their family or friends,