@Erin9210 This sounds very wrong and unfair.. I am sorry you are dealing with this on your holiday.
Was this booking made via Airbnb? You state you rented from homeowner. If it is through Airbnb this is not allowed. You can call Airbnb customer service and I am sure they would echo the same.
If not through Airbnb, I would not allow entry for showings if I could help it. The 24 hour notice you mention is typically for "tenants" and not transient rentals. Was there anytype of rental agreement signed? If so there may be some language in there about entry however it's usually reserved for emergencies such as plumbing and mechanicals.
I wish you luck and hope you can get this worked out.
Karen