Fudwinkel,
You're kind of on the right track. You can convert your ATP to ATPL with the single air law written exam (provided you have an original or converted CASA CPL and a CASA CMEIR). What is being said is that some airlines want you to have completed all 7 ATPL exam regardless of CASA reqs. You CAN sit the other 6 exams after you have converted your FAA ATP to CASA ATPL but the catch is, if you fail one exam, you lose your ATPL and go back to CPL until you pass every exam. VERY ANNOYING!!! THats what I've been told and pretty sure duxnuts will back me up.....
Where you trained is irrelevent to the airlines reqs but will make the CASA conversion flight test/s more difficult if you haven't lown in Oz.
Just read that CASA is making changes to their licencing which includes a ATPL flight test. Please note that this could take CASA 20 years as they are government run.