Having been on two ex-CNS liveaboards I can assure you they will have you out and off as soon as they can. The turnaround time is very short and they need to get the boat ready for next pax. I also did the CNS-MEL flight but round availability very tight. I was waitlisted until the day before.

I agree with what has been said about giving yourself an extra day and removing some of the stress. Hotels in CNS are relatively cheap. I stayed in a small motel type place and it was excellent value. Good sized room that was clean and comfortable.