1/ Good
2/ Tracking over water not uncommon to use airspace boundary as route. Similar to the old DME Arc.
Cannot say for sure that was the crew intention here.
Or, could be as simple as tracking Anna Bay - Bankstown direct.
The reason I queried your night experience was your statement -
"How do you follow the coast visually in total darkness?"
Mr Smith, I hold you in high regard, and respect what you are trying to achieve. Our views regards airspace organisation are somewhat similar.
However, I think you are barking up the wrong tree on this one. Two or three options as to what went wrong here. Airspace was not one of them.