If she were able bodied I'd say book virgin on the same PNR as etihad flight. I fly NZ into brisbane then onwards on VA quite a bit and once you've collected your bags and cleared immigration and exited there is a transfer desk to the right. Dump bags, they seem to reissue my boarding pass despite being printed from NZ already then I take the free bus to domestic, get off and go straight through security (haven't found the lines long to date for VA) and straight to lounge or gate.

There is also a train and with the alliance with NZ/VA you get on free, just show tickets VA issue. Last time I did this I was told the train was running quicker than the bus. When I took QF and waited for the bus it seemed to take forever to come.

I'm presuming the EY/VA alliance is similar to NZ/VA. Maybe you should check with the VA forum. For me I would book that connection. If it didn't work out due to delays it would be on the airline to sort.

Your mothers mobility issues do make it tricky. All you can do is call etihad and virgin to see what their advice is on what they offer assistance wise and run the plan past them. An alternative - do you think you could meet her in BNE? Fly up for the day?

If all else fails, find the schedule that works, then have her make her way to the VA lounge and if she's up for the $65 could pay to get into the VA lounge to kill some time if you're planning a longer layover.

If your mum has a smart gate passport that will help speed things along. I think calling the airlines is the best way to sort it though.