Good questions. Firstly, I said progressive creation fits the fossil record better than evolution - I didn’t mention biogeography. As a recent convert (only about a month) to progressive creation, there are no doubt many questions I haven’t considered.
Regarding marsupials, were there not fossils of kangaroos found in China? Your evolutionary explanation could also be applied to progressive creation, which doesn’t preclude the forces of natural selection.
Furthemore, I have no problem with God creating creatures in only one part of the world. I don’t presume to know what God would or wouldn’t do.
The ‘isolated islands’ you mention may not have always been isolated.