1) Perhaps try BCom with Engineering breadth, then if you realise you want Engineering, then you can proceed to Master of Engineering. Having a good foundation on business could be help you in your Engineering career. If you decide that perhaps Engineering is not for you, then at least you're in BCom and can apply for corporate/non-corporate graduate positions right away.
2) Yes (
http://bcom.unimelb.edu.au/breadth/breadth_studies_in_engineering). Ask the Commerce Student Centre.
3) Depends on the major. Actuarial grads can go into investment fund companies, superannuation, banks, government, etc. Accounting grads can have careers in assurance services, financial reporting, controllership, consulting, advisory, taxation, banking, and government. Finance/Economics grads can go into corporate finance and restructuring, risk services, economic analysis, investment banking, financial advisory, treasury & wealth planning, academe, government, etc. Management/Marketing grads can have good careers in corporate/brand marketing, market research, operations & supply chain management, HR, and advertising.