Well, you've always got university pathways, so I guess your preferences should go:
1. Law/Commerce @ Monash
2. Commerce @ Melbourne
3. Commerce @ Monash
For option 2, you'll just have to do well in university and hope you can get into the post-graduate degree (most likely, you shouldn't worry about this too much). If not, then you can always go for Monash's post-graduate degree, or the undergraduate one even.
Option 3 probably wants a transfer to UoM asap, or a transfer to Monash's Law/Commerce.
Edit:
About the art units, it doesn't matter, you just might not be able to get credit for it towards your degree (which is trivial, because you would probably only get 12.5 points of credit -- one subject worth -- which just means you do one subject less than everyone else, and that might actually limit your exploration in your BCom. degree)