If you don't make the cut for medicine, then you'd probably be forced into med science anyway. I don't know what your parents are like, but maybe just carry on and do the electives you want in med science, focussing around what you want to research... convince them that Honours is a good idea as it will boost your career prospects and then just leg it into a PhD when you graduate.
The thing is, do you want to research the disease, the pathogenesis/organism, or be treating patients who get it (obviously that would be waaay down the track)? I guess it depends on the disease and what you're interested in. I'm from the med science camp, doing work with dengue virus, I never see people; I am only looking at a single viral protein and the way it interacts with host proteins. You don't see people who are experts in every aspect of a disease, you'll find that people are kinda like this....
actually, looking back at that picture, I would say the green and blue sections are a lot smaller!!!! I guess what I am trying to say is that if you want to research, and not have to go though god knows how many years of medicine, spend time treating people (where you won't learn the molecular biology skills to research as a PhD would), just got straight into PhD after Honours... if research is what you want, then no amount of additional undergrad will make you look better, because once you pass that undergrad boundary, they care about your
publications, their quality, your skills as a scientist and your research experience.
And in regard to limited job opportunities, that is only for people who don't plan their degrees properly and who think they'll just be given a job... often you'll find they are the ones who whinge the loudest; primarily because the ones hard at work don't have time to whine. if you have the dedication, you'll be fine. I'm from a small (typically viewed as backwards) university, and have heaps going for me as far as my future is concerned (if I do say so myself lol)... and its because I worked my butt off and continue to do so.
But chin up, don't burn out in your HSC, because uni is going to be a 7-8 year slog which is far more intense than your school years (because you also have life/the real world to deal with).