Yes it is possible to go on to do JD from Science or Biomedicine. Yes you could choose to do both the LSAT and GAMSAT if you want to keep your options open and decide which career path you want to pursue later on. The best way to approach the LSAT and GAMSAT is to do it multiple times during your undergraduate degree due to the two year validity of the results. I did the GAMSAT three times: Beginning of 2nd year (GAMSAT AU), Towards the end of 2nd year (GAMSAT UK) and at the start of third year (GAMSAT AU). From that I selected my best result and that was used towards my application towards Dentistry. I would imagine it would be the same for LSAT. Both LSAT and GAMSAT are general ability exams. Except LSAT would have more of a focus on what GAMSAT has in Section 1 and 2 (Analysing Texts, Logical Reasoning, Reading Comprehension and Essay Writing) and not much of the Science Section obviously. University is busy, but you will be able to make time to study for the GAMSAT. But in saying that, it probably would be best to do it at the start of the year so you have the whole Christmas holiday to prepare. However, unless you're confident, I'd do it three times. I guess what I'm saying is there would be quite a bit of overlap between studying for the LSAT and GAMSAT.
Well if you get confused looking at the breadth subjects that you're head hurts then a Student Advisor is probably necessary. There isn't any subjects that you are supposed to choose for breadth. I chose subjects that would give the best chances of getting a good GPA and breadths that I found interesting and enjoyed. That is the whole intention of having a breadth component. However, if you love your Biomedical/Science subjects that much then there are 2 'free' breadth subjects where you could choose a Science or Non-related science subject to study (The other 4 allocated breadth subjects have to be from a non-science faculty). (My Breadths were: Managing and leading organisations, principles of marketing, genetics health and society, gamelan ensemble, music psychology). Yes, breadth subjects count towards your GPA at equal value to all your other subjects. You are able to do three level 1 subjects. So I would save 1 or 2 level 1 breadths for your later years when they are weighted more. Since your first year and third year are weighted x1 and x3 respectively.
Hope this helps!