I can only imagine how horrible the whole ordeal is. You're supposed to return from a vacation refreshed and relaxed, but this sounds like a total nightmare... and then to have to deal with bad guests.
Keep repeating: "this WILL be behind me... this WILL be behind me" (that's what I do when things get particularly tough). Project yourself into the future as best you can and imagine you're looking back on it after it's "one of those bad stories" you tell.
Just a thought... if the guests left things contrary to the way they were supposed to, would that not offset the sighting of a millipede? I mean... you're in the tropics.
Funny story: once we drove to the Grand Canyon and stayed in a single-floor motel in Flagstaff. During the night I heard something so turned on the light to see a millipede had knocked over an object. He was HUGE! Like about 5" long (12cm). I was freaked out (we don't get those in Canada) but also so intrigued that we caught the guy, named him and brought him home with us.
It wouldn't occur to me to ask for my money back! A normal person would know that is out of the control of the hotel. ... but especially when they hadn't followed your instructions? C'mon - it goes both ways.