Regarding being allowed to sit the ATPL exams if you allready hold an Australian ATPL (by way of of doing the Conversion Air Legislation Exam) all I can say is what what was told to me by the Casa Lilcensing & Registration Centre (CLARC) when I rang for a mate who was overseas.
Over the phone I was told that if you hold an Australian ATPL you can NOT sit the subjects. This was backed up by way of email.
All I can suggest is that before you believe me, or anyone else on this forum, you contact CLARC on ph 1300737032.
They are very helpful and if one day you want to fly for an Australian company that requires you to hold all the Australian ATPL subjects (not just have the Australian ATPL) you do yourself a favour and find out first hand.
Good luck in whichever way you go.