@Michael-and-Abi0
I thought I'd heard it all by now, but this is a new one to me. What weird behaviour.
I do hope you have the guest's threat of a bad review in writing, preferably in the Airbnb message thread? Even if not, I would still contact Airbnb CS and explain the situation, i.e. that the guest tried to extort you for money with the threat of a bad review.
You can also ask for their advice as to what to do with the mattress toppers etc. I wouldn't be happy with these guests coming back and would want nothing more to do with them. Seeing as they have already carried out the threat of a bad review, there is no need to appease them further.