@Melody22
I wish there was a way you can tell the guest that if they decide to proceed with their lie about your listing just to get you into trouble, that they would legally be on the hook for any damages you sustain. In this case, you would add up the value of all the future reservations you have upcoming and state that you will take legal action to recuperate the value of said damages including the fee to cover your legal fees (lawyer, court fees, travel, etc). If you do this, make sure you actually add up the value and put that value in the email to make them realize the potential consequences of their lie.
As a guest, If I was so devious to do this to a host, I would instantly backtrack and not proceed with any more lies to airbnb because I would realize the legal implications for me and also realize it just isn't worth it to get back at a host just because they caught me in breaking a house rule.
Unfortunately, I am not sure how such a statement to the guest would be taken by airbnb but it is something I would consider because the fact is, this guest can easily frame you but falsifying photos of cameras in your home. If they are already demonstrating that they are lying about your listing having interior cameras, I wouldn't be surprised if they took it a step further and falsified images.
I hope this works out for you. I can tell you I have had false accusations before to by some very vengeful people and it can really make you lose hope in humanity but you also have to protect yourself.