I think Melbourne is a wonderful place for you to do your undergraduate degree in either Science or Biomedicine. That's not to say RMIT would be a bad choice, it just gives you a good foundation and understanding of some medical related area (if that's what you're passionate about) in an academic sense. I've really enjoyed my time at Melbourne!
If you're serious about doing medicine, have faith in yourself. Choose what you think you will enjoy more to do in your undergraduate degree, rather than something purely vocational. If you think you'll really enjoy medical imaging then that's great, go ahead, but the way you phrased it is that you're just looking to get a job out of it. The fact of the matter is, even with a Bachelor degree from RMIT, you probably won't be able to get a job straight away...it's something a lot of Year 12s are going to need to start learning. Gone are the days you could safely walk out even with a vocational degree and expect jobs to be yours for the taking. My brother works in medical imaging but he has a graduate qualification and he only just finished his proper training and qualifications at the hospital he works at. It's not as simple as 'having a job' straight off the bat in any profession an I can imagine it will only get worse, everywhere.
You might find that university suits you more than school. You can take the GAMSAT three times during university too prior to graduation and another couple of times if you decide to add an Honours year. Even if you don't do well the first time, there is lots of room for improvement - unlike the UMAT. Try not to stress too much about whether you would fail because even if you do, there is always room to improve your marks.
I don't really think you would fail the interviews - you might miss out on your first preference based on a poor interview but you probably wouldn't miss out entirely just based on interviews...someone correct me if I'm wrong though I'm kind of clueless at the interview bit LAWLZ.
Lastly, if you're aiming for the MD at Melbourne in light of your UMAT score and the fact that you don't think you're eligible for Monash (just because most people do) it is good to at least find out if the medical imaging course you'll be taking (if you do go down that road) has the proper equivalents to Melbourne prereqs for the MD or not. You can look that up on the MD applications website but it's something you should probably consider if you will be aiming for Melbourne. Difficulties such as those are removed when you choose a Melbourne bachelor.
I think you should just decide what you want to do more based on your passions. Will you be fulfilled in a medical imaging job? You might find that you benefit from a broader undergrad degree if you're not sure.