Great response! Could you touch on question 2 more (in terms of GPA and if its easier to get in)?
I'll start by explaining what GPA is for those that don't know. When you study at Melbourne (and most universities in Australia), you get both a numerical grade and a letter grade. At Melbourne, a score between 80-100 is a H1 (first class honours), 75-79 is H2A, 70-74 H2B, 65-69 H3 and 50-64 Pass. Most university students are simply interested in the Weighted Average Mark (WAM), which is exactly as it sounds.
However, for certain courses, they use GPA rather than WAM. GPA is calculated based upon the types of grades you get. It varies between universities, however, at Melbourne, a perfect GPA is 7. It is the average of the 'points' that you accrue depending on what grade you receive. A H1 = 7, H2A = 6.5, H2B = 6, H3 = 5.5 and Pass = 5. So for GPA, at the end of the day getting 80 or 100 doesn't matter, whereas getting a 79 vs. and 80 can be a big deal. Usually, when calculating GPA for medicine entry, first year is worth less than second year which is worth less than third year. You can read about this in the entry requirements of the courses.
In general terms, the university will try to make sure that there is a fair distribution of grades. For example, they generally like there to be around 20% of students getting H1's. In the biomed cohort, most people have the intention of wanting to do medicine, so they are aiming for H1's. In science, since the entry requirements for graduate courses varies drastically, you end up with students who just want to pass or meet the bare minimum. Thats not to say that there aren't students in biomed who are the same.
GPA is only one component of entry. For dentistry and medicine you also need to sit the GAMSAT. If you want to know more about that, I believe there are some good threads. For medicine, you also need to sit the multi-mini interview.
Ultimately, you can't really say if biomed or science is 'easier' or if one is going to help you get into med over the other. I have found in my experience that the biomed cohort is much more competitive when it comes to assessments, which means they are often made more challenging than science due to the need to achieve a good grade distribution. You might enjoy being in a more competitive cohort with a little bit more structure. Thats not to say that science isn't challenging either, I majored in chemistry and they really know how to challenge students.
My advice is to pick the course that you think you will enjoy the most and pick subjects where you will be challenged and learn interesting and important content. If you enjoy a subject, you generally will do better rather than picking a subject. You won't realise it at the time, but you'll look back on your undergraduate and there will be subjects that you wished you took.