Cancelled flight 2.5 months ago - still unresolved and out of pocket

We spent 12hrs inside Cairns airport Christmas Day to be left over $1000 out of pocket and 40hrs late to our destination. What gift!
Flight out Cairns to Osaka cancelled due to engineering issue. Line to rebook flights and organise accommodation approx. 6hrs long with 3 Jetstar staff working. After in line for approx an hr receive email from Jetstar telling us we can rebook flight via link free of charge. Great! click link - no options available. Announcement over PA "we suggest organising your own rebooked flight or risk spending an 3 additional nights in Cairns waiting to get a seat on a new flight".... Shit! So we book a new flight at a cost of over $2000 assuming, as we have in writing from Jetstar that our new flight would be free of charge, we were entitled to a refund.
We have been back and forth with Jetstar for the last 2.5 months and so far have only managed a refund for the original cancelled flight but have been told we will not be given a refund for the difference in fares. Over $1000 difference.
To rub salt into a wound our flight home was delayed by 18hrs stranding us again in Cairns for a night and causing us to miss a day of work.
We have logged a complaint with Airline Customer Advocate and are trying to claim back via insurance but so far no luck.
They are succeeding at wearing me down but I will not let this defeat me. If anyone has any advice on what our next steps should be would really appreciate it.

Japan-Osaka (Kansai International)