@Terrill0 don't give them a cent back! You've been crystal clear about your cabin, and everything it does and doesn't have. (I'd love to come stay!!) This is entirely on them. People don't read, and you shouldn't have to placate these non-reading fussy guests with money to make everything better.
Ummm....maybe take your socks off if you can't climb the ladder in them?
If they decide to play dirty and turn the possible creature sighting into an actual mouse sighting, that might make trouble with AirBnB, but they have no proof.
Just be happy that they'll never book again, and hope that they decide to go away and stop bothering you for a refund.
You have a lovely place and many happy guests and good reviews. Just keep doing what you're doing and don't worry about this.