Hi, have done multiple flights on Etihad both business and economy with infants, the crew have always been super helpful. I don't know if bassinets are available during Covid or not, if they are book one, using them is based on weight, 10kg I think and if they fit they work well, they always provide multiple blankets etc to make it comfortable. Otherwise book the two middle seats this works ok as well, the barrier between can be lowered to almost seat level and its easy to pass the baby between etc. Also since the flight wont be full, it is likely you will be able to use one seat for your baby to sleep on, and you take a spare seat, we have done this a few times and it works well with a few extra pillows and blankets around the baby on the spare seat.

The kids play area is not really for 8 months old, if it is even open, book the hotel inside the airport since who knows in May if you will be allowed to leave the airport, as of now you will not be allowed to leave the airport and 9 hours in the lounge is a long time with a baby.

Also keep in mind it is highly unlikely that the caps and quarantine in Aust have changed by May so the entire flight from AUH to MEL will be practically empty and since Etihad have the full schedule loaded in May, I would imagine that the flights will also change at some point in the next few months.