We ended up using the suitcase option and it worked fine. The plastic bag was great for the umbrella stroller.
The car seats themselves, however, were a disaster. Due to a recently changed (1 March) law in Queensland, our seats were illegal. All foreign seats are illegal and, according to a driver, a good number of Aussie ones are illegal.
Seems Australia is going the way of the USA, every state with a different standard. You other countries need to learn to not follow the Americans everywhere. A lot of Americans have no idea where they are leading you.
In my common sense view, seats from a reputable manufacturer and a reputable country should be allowed in other reputable countries. You can't have a car seat for every country or every state in a country. Seems car seat manufacturer/rental lobbies are too powerful and not enough politicians have kids.