Wow, this has been a long post for such a short time with lots of people GUESSING as to the problem.
I have my own GUESS but that's all it is so I won't air it.
I GUESS that some of the symptoms could be attributed to OPVM and then again many other thing could cause similar signs and symptoms.
To clarify, an autopsy WILL NOT, repeat WILL NOT in most cases give a cause of death.
An autopsy might show death from cancer, severe R.I. and some other thing, but when the is nothing obvious, tissue samples MUST be sent to a pathology lab for testing.
This is THE ONLY WAY to isolate things such as viruses, other than doing that it's only educated guessing by a reptile vet.
Now given your distance from Sydney or Adelaide I'm not sure if tissue testing is an option so take care with your diamonds.
With regards to the green staining in the poo... I see that most winters with my bhp. A few years ago I started asking questions. The most consistent answers came from a reptile vet and a vet school.
I only see it during winter when the bhp's aren't feeding and to survive the 3 or 5 months they don't eat they have to live off their fat and tissue reserves.
To metabolize these reserves they produce more bile (green) and what come out is just excess bile.
Sorta makes sense as I never see it during summer.