Quick Islay update for those whose sailings have been cancelled and rescheduled.
If you phone Calmac, it's possible to get wait listed for the earlier return boats (9:45am and 3:30pm on Monday 7th, I think) without affecting your confirmed booking. If someone with a booking for those sailings changes their plans, then Calmac will apparently go to the waiting list first and you are then moved from the waiting list and given a confirmed booking for that earlier boat (replacing your existing booking on the late boat).
Alternatively, there is usually more capacity on the boats than confirmed bookings so it's also possible to just turn up and queue and hope for the best.