I don't think you can combine those two (monetary wise). You will get the higher of the two (i.e. 16k). However, you still get recognised as a scientia student and get the non-monetary perks associated with it.
Does the Pre-Law Program guarantees a spot in the BA/LLB program next year? If so, just do law at Macq. As for the UNSW route, a distinction average in first year is definitely achievable but again it depends on the individual.
Law isn't that prestigious at all and as for the pay it's not that great. It's a highly competitive market out there - some people apply for 30 or more firms - culled at screening stage. You have to be a close to top student with an excellent CV (extra-curricular, relevant work experience &...