@Mike1034
(1) I would reject the request, but at the same time ask that the guests follow the cancellation protocol and uphold the terms of the cancellation policy that they contractually agreed to.
(2) Yes, I would ask for the guests to cancel, because even for a one-night booking, you never know when you'll need documentation of the time and circumstances of the departure.
(3) No refund except that which is automatically granted by your cancellation policy. That is, unless the guests left because you failed to provide all the advertised amenities in clean condition. (You have a great record so I doubt this is the case)
(4) I don't think 15 minutes is enough basis for a review, so I would only write one if I get notification that the guest submitted one first. Then, it's basically butt-covering mode. The review should plainly state that the guest departed without explanation but chose not to cancel their booking.
The star ratings are pretty insignificant in the host-to-guest direction, and it's ridiculous that you have to rate the tidiness of someone who didn't actually stay there. But if I'm not allowed to publish a text review without filling in those stupid stars, I guess I'll go 1* straight down the board. 1* forcommunication (failing to supply an accurate guest count or explain the early departure), 1* for following house rules (cancellation policy is very much a rule), 1* for cleanliness because that refund request made me feel dirty. 😉