This is probably a bit late for the OP ...
I too live in Brisbane, Australia. I had an Outlander PHEV from 2014 to 2017 under an operating lease. I declined the option to purchase the car at the end of the lease and I returned it back to the leasing company.
In the three years I had the car, the battery capacity dropped from 10kWh (when new) to 8kWh at the end of the lease. This is around 30km range on battery. I tried a number of times to engage with Mitsubishi about the declining battery capacity, and they refused to acknowledge any problem with the car.
In general, the car had an easy life, trundling 40km through peak hour traffic to and from work each weekday. Occasionally, it had longer runs in the country.
I agree with you that one really needs air conditioning in vehicles in Brisbane in summer (which is effectively late October through to early April). It is not only the heat, but the humidity that is oppressive in January and February. Even when it is raining, the humidity is so high that one needs the AC on, just to dehumidify the cabin so that the windscreen doesn't fog up.
From my perspective, the battery capacity in my PHEV started to decline after the heatwave in 2015 and again in 2016 (when we had a week of temperatures regularly over 40C and approaching 44C at times). Perhaps the same degradation happened to yours as well.
In terms of next steps, I think you are going to have an uphill battle. Base on my experience with Mitsubishi, they will be very unresponsive and uncooperative. I suspect getting any battery repairs done will involve lots of complaints to various government ombudsman and consumer affairs organisations. There might be some benefit in getting legal advice in launching a small claims action against Mitsubishi.
Two other bits of advice that might help. First, carefully read the traction battery warrantee in the owner's manual. Mitsubishi Australia's warrantee back in 2015 was not as good as what Mitsubishi's offered in other countries (e.g. UK). Second, I would try to dig up any old Mitsubishi publicity material from 2015 to see if there was any "implied" statements about the expected battery life.
The other options include trading the car in (good news in the current car market is that vehicle prices have held up extremely well) and absorbing any financial loss. While technically possible to replace the battery, I don't think this makes financial sense. Or, finally, just living with it as there are still fuel savings to be had.
Best of luck with getting some warrantee work on the battery.
