You're plight sounds awful and Airbnb's response entirely unreasonable and inadequate. There's nothing I can add to the sound advice already given by others for dealing with your dilemma, however, in the future, you might like to consider these tips to prevent a reoccurrence.
1. Establish exactly how many bedrooms the guest has 'purchased' and lock the doors to all others.
2. Have an additional guest fee that applies to any guest not included in the original booking headcount. Make this fee high enough to be meaningful and make its application a House Rule. In my case I call it a 'surprise guest' fee and it is $100 per head, per night. This defines the amount to which you are entitled and for which the guest is liable.
3. Define the circumstances under which a 'visitor' becomes a 'surprise guest' and therefore attracts this fee. In my case, I define a 'surprise guest' as anyone not included in the original guest count who is on the premises for any period between 11pm and 8am.
4. Include a code phrase within your House Rules which the guest must quote to demonstrate they have actually read the House Rules. If you use Instant Book, include the requirement to quote the code phrase in your pre-booking message. Make this code phrase 'Surprise guests are expensive' or something similar. Yes, I know that guests tick the box to attest to acceptance of the House Rules but, as we all know, they never, ever read them unless forced. Even though the House Rules are still applicable and (ostensibly) enforceable regardless of whether the guest actually reads them, it makes the conversation a lot easier and less prone to argument if you have evidence that the guest was made aware of the rules and that a particular rule designed to curb problematic behaviour was emphasised. In my case my code phrase is 'I know it's not a party house'.
5. Install exterior surveillance cameras that upload to the cloud in real time so that (a) activity is recorded and (b) evidence of tampering is captured and stored. I use ArloPro.
