I try not to leave anything up to my guests to remember to do or make their responsibility. It's just a resentment waiting to happen if they don't do it and their thinking is that they are on vacation..if they had to remember stuff they'd have stayed home hahah.
I use synthetic wicker furniture because it's very comfortable to sit on without cushions and is waterproof. I also use outdoor, weatherproof cushions because I live in Florida and it pours, sometimes daily. Weatherproof cushions make it a non-issue. You don't have to buy the expensive waterproof fabric Sunbrella, there are many, many waterproof fabric cushions. If you want to keep your cushions what about putting them in the dryer?
Second point..if a guest does something that creates an uncomfortable situation for the next guests you can request payment for the situation. If they ignore the request or refuse to pay AirBnB has always paid. I had a guest smoke in one of my apartments. THEN she used a horrid perfume to try to cover up the smell. I didn't have any guests booked for a few days but I also had to take it off the market for two days. I requested the nightly rate for two nights as damages. She refused, AirBnB paid.
Bottom line it's so much easier if you have room to store backups of these types of things. Also, for another rental booking site I use they tell you to put it under house rules. That way, if they don't comply with what you've asked them to do regarding taking the cushions off before rain then you have recourse because it's in your guest rules.
Hope this helps