I wouldn't rely on any spare seats being available particularly if you're travelling to Hawaii in high season.
Looking at airline seat maps when you online checkin will only show you seats that have been pre-allocated and/or people who have already checked in.
Any 'empty' seats does not mean they will stay that way. It could be a full flight so all those seats that are unallocated when you are doing OLCI will be allocated to travellers when they checkin at the airport. The airline won't really know exactly how many seats are left over, if any, until they close the flight for checkin which will be about one hour prior to departure.
As always you've got to assume the worse case scenario ie no upgrade coming through & your husband stuck in economy with no spare seats next to him.
On almost full flights any spare seats will generally be scattered middle seats throughout the aircraft not rows of three or 4 seats together suitable for lying down on.
Have you considered travelling on NZ ex AKL to HNL? You'd need to fly to AKL the day prior but it is a day flight 1100/2145-1 & they have special J fares on
www.airnewzealand.com for approx AUD1,800.00. I think at least one adult family member should accompany him in a carer role. Coming back the NZ flight is an overnighter so you may be better off taking a day flight HNL/SYD on QF, HA.
You absolutely must contact the airline re medical clearance and obtain approval to travel & they will need to liaise with his treating Doctor.
The other hurdle will be trying to get travel insurance but it may be worth calling Iselect who can do comparisons with the different travel insurance companies.