Thanks @Gwen386 . As you can see, they are ganging up on me. Like I said, coordinated, unlike a lot of CS reps.
As for this particular situation, it's a tricky one. It sounds like @Fergus32 only started to become uncomfortable with this guest after the booking was accepted (or maybe it was an IB, I don't know), because of their constantly changing plans and contradictory accounts. It could just be that the guests are disorganised, but claiming to have COVID to try to get a refund and then changing the story would also put a bad taste in my mouth.
I had a slightly similar situation once. I was able to check the guests in on another day as I'm a live in host, but they changed the date and time several times, usually giving me very short notice about it, so it was frustrating. The main thing though was I was sure the guests were lying about having COVID (their excuse for the second date change) as the story just didn't add up. Once I made it clear that they wouldn't get a refund for the nights they missed, they were suddenly ready to check in the next day, to which I said no, sorry, you cannot come to my shared listing with COVID! You have to complete a quarantine first, send me negative test results etc. So, they shot themselves in the foot.
I did feel uncomfortable about hosting these guests because, if they lied about that, what else were they going to do? They turned out not to be the worst guests ever, but they were certainly problematic in several ways and I would not host them again. Did I have enough reason to cancel their stay though? I'm not sure. I had a hunch they were lying (and disorganised), but I don't think that was reason enough not to honour the booking.