The Antarctic overflights are treated as domestic, but then they don't land. Since no country owns any part of Antarctica (by treaty) I don't think it would qualify as domestic, and since not landing in a different country (Antarctica isnt a country in its own right) it isn't an international flight either!

Point of no return - I believe so. While there are a small number of airfields on the coast of Antarctica (and even smaller number inland), there is no practical alternate airfield for the flights once they get much south of Tasmania and NZ. IIRC if an a/c heading for Antarctic base cannot land at that base for whatever reason, they have to head to another Antarctic airfield and hope conditions there permit a landing. That is one reason (of several) why there are no mid-winter landings at Antarctica - too high a risk of having no safe airfield to land at.