@Caitlin20
Would it be those stating in their review that they were a group of 8. If so, you have solid proof despite your property being in Spain and yourself in the UK.
I would send a claim for the extra guests provided the extra guest fee was in place at the time of booking.
As for leaving a mess, the only thing you can do at this stage is mention it in the review.
Seeing your property attract hen parties and the like, you would need to have clear house rules on how to leave the place on departure.