Another thought: could she start an IB program in Australia now and finish it at a public high school, thus giving her a recognized qualification for application to regular four year colleges at the age of 17 or so? I know that one of the problems is the mismatching academic calendars, but you were planning to take a semester off for the move anyway, right?
Community college for two years and then transfer to a four year college at the age of 19 or so sounds like an interesting option, too - again a way to give her qualifications US colleges are familiar with. I'd worrying a bit about finding the right sort of challenge.