Cdp,
I understand your predictament.
I too commenced a law degree and then opted for an aviation career.
I have never regretted my decision and have friends in the legal profession earning less and working much longer hours.
WRT Qantas pay. Initially it is between 70-100K for the first 18 months and then kicks up substantially to around 130-150K. As mentioned, it does depend on the fleet you are allocated to and the allowances that you save. I know guys who save $1000 per month (cash) in allowances alone. (I personally save $1 - hmmm).
Communting from Melbourne is easy. A very large number of guys do. You can use staff travel to fund the flying, however Qantas also allows you to offset paxing sectors to achieve this (long story). The point is, you will not be paying for every MEL -SYD-MEL trip, only occasional ones. (It gets easier the more senior you become).
My personal opinion , as was mentioned, is follow your heart. A qantas cadetship is a prized achievement. It is a much more attractive proposition than bashing around GA for 10 years earning 25K pa in clapped out C210s.
Hope some of this helps.