Thanks for the replies! Especially the tidbit on booking ADL-MEL/MEL-ADL on separate tickets, I hadn't even thought of that! I'll definitely do that. ^

In looking at the flight history of QR914, it seems mostly on-time. But as we know, past performance is not necessarily an indication of future results:
https://flightaware.com/live/flight/...100Z/OTHH/YPAD

Just for piece of mind (and the fact that most Airbnb's have a 10am check-out), I will probably just book the 1pm MEL-ADL, arriving around 2pm. I could store any bags and go into town and stretch my legs, and have a decent meal before the long flights (plus I've never been to Adelaide). It looks like there's luggage storage at the airport:
https://baggagestorage.com.au/locati...laide-airport/

For the ADL-MEL, it looks like 7:25pm is the latest QF (or any) flight out to MEL that night. If it looks like the QR flight is running very late (as in hours), I can proactively get online in Doha and look into a hotel in town and a flight the next day. It looks like QF has 8 flights/day. That would be a Friday, in January, so it is possible things could be busy, but I'll keep an eye on things and hope to grab something if needed.