Yes, you will be able to apply for graduate medicine at melbourne uni so long as you undertake the required prerequisite subjects at second year level. Biomedical science and law are both quite demanding in workload so you will have your hands full. You can transfer to the single degree but grades will still be taken into account in the transfer. Should you be successful you will gain credit and do second year subjects. Though i believe you will have to take a few subjects in law that would otherwise be taken in first year for pure law students (i am not 100% on this as i dont study at monash law school).
There is no preference for selecting Melbourne uni biomedicine students to any other biomedical science degree, or any other degree for that matter. So you will not be disadvantaged by studying at monash.
I'll leave your last question for someone else to answer, but from my knowledge there is not a great deal in terms of difference other than prestige. You can basically do everything the advanced students do in the mainstream course.