In your situation I wouldn't accept the booking as these guests sound like they will be demanding.
I would reply to the guest and say if they need to cancel because one of their party has COVID and provides evidence of this they would eligible to apply for a refund under Airbnb's EC policy for COVID.
Please make it clear to the guest that if they cancel for any reason not covered by Airbnb's EC policy any cancellation will be processed under the cancellation policy they booked under and they would not be able to rely on getting a full refund.
I would never use a flexible cancellation policy as it is too risky as guests can cancel at the last minute and you will end up out of pocket if you cannot rebook.