1/. I suspect CASA is the best source of information on this one - sorry.
2/. The fact that you have an overseas ATPL and previous complex turbine experience will make you stand out in the pilot market - just make sure you get all the other pre-requisites that CASA have stipulated;
3/. If you need an ATPL for the job, they would surely include the test for that into your type rating training (all the more important to get the pre-reqs nailed)
4/. Remember that most youngish Australian pilots are in the same boat, with the last CAR5 ATPL handed out some years ago. They all have to get the hours and pass the exams, and get a job as an FO, before getting command upgrade and doing the ATPL test as a part of that process.
The days of the "GA ATPL" pilot are numbered.