For international seat release bookings, where the domestic connection is knocked back by the robot...
The written policy is that the domestic connection can be manually forced in as long as C class is available for J, or S class is available for Y, on the domestic on a standalone basis (eg...
This depends on whether your source of avios is buying them or earning them.
For example, if you bought the avios, in many cases using the lowest avios amount and maximum cash can be the best value depending on what the value is that you assign to the avios, because you stretch your bought lot...
For QF, the equal or higher rule is based on pure dollar figures. QF don't restrict changing between cabins.
Some airlines however have rules where it must be equal or higher AND also equal or higher RBD (ie. subclass). Eg. if your ticket was in D class, you can only go to D class or higher and...
AA scrapped that and fired Vasu Raja over it too... heads certainly rolled, as the industry long expected when that change was initially announced. AA realised eventually, albeit a bit late, that you don't mess with the travel trade.
You qualify by virtue of having an international ticket that bookends the domestics, irrespective of how long you stopover. Basically if you have an international ticket... (that can be train, ferry, cruise, flights...),, you're covered,
For more expensive domestic fares, you can exempt two taxes on the domestic ticket if you also have an international ticket.
You have to purchase from a travel agent that knows how to do tax exemptions and they must be located outside of the US though, but in your example you could probably get...
For QF direct bookings, if your ticket was worth $500 and you book a new fare that is lower, eg. $200, then the $200 fare will use the existing PNR and a new PNR will be created for the remaining $300 credit.
For agency bookings... you can just re-use the same PNR over and over.
I have seen this incorrect information being sprouted for quite some time now... so for the sake of getting it right...
For QF flights, QF sets the fuel surcharge. You are correct that these go to QF's bottom line. However, for non-QF flights, QF doesn't set the fuel surcharge at all.
Carriers...
If you know your passports are already linked (the link lasts for the life of the foreign passport, even if you get a new AUS passport the links are automatically carried over), never hand over both passports, only the one you will enter the destination with.
The fact you had to use the AUS...
The correct process is you use the passport you checked in with in the smartgate, so that means you use the USA passport in the smartgate, not the AUS passport.
Something is not adding up here... SQ is probably the only major carrier who does very few schedule changes and certainly cancelling flights altogether is not something that SQ does often. So why were the flights cancelled?