@Dario267 There's no action you can take that will buy you a 5-star review. Even if you let these guests get away with murder they might still come at the end demanding refunds over some fictional problem, and even if you throw money at them they can still give you a bad review. This "5 stars" thing is absolute garbage, and you'll make the worst decisions of your hosting life if you cave in to it.
Much bigger concern is the fact that you don't know how many people are in your property, and that the one thing you can be certain of is that they are liars with no respect for you, your home, or your livelihood. In hindsight, of course you should not have given them the keys until the guest count was adjusted and the extra fees fully paid. You'll be ready for that the next time it happens.
For now, you have a Trojan Horse booking, and I'm sure you know how that worked out for Troy in the end. You're the boss of your property, and this is one of those occasions where you're going to have to step up and act like one. Start the paper trail with Airbnb by communicating through the site that you need to amend their booking to account for the total number of people in the property. (This is true - if there are unregistered guests present your Host Guarantee is effectively null and void). If they are uncooperative with your efforts to secure an honest accounting of what's going on in your house, you need to be prepared to remove them from it.
You've already lost a lot of leverage by backing down on the extra person fee, so pushing that issue is probably a lost cause. But your home is being occupied by people who think you're too afraid of a bad review to defend your own property, and you're not likely to get much from Airbnb if they trash it, so it's really up to you to make sure they realize you're not "closing one eye" anymore.