I think you are really mature to allow yourself to consider both options here; most people who are absolutely sure about medicine will try to jump into the undergraduate pathway, perhaps without thinking what the graduate pathway would entail.
I don't think anyone can ever be 100% on medicine (or any career, really) straight out of Year 12 and if someone said that they were, they're either in a really special circumstance in order to make that call, or they're lying. In reality, we have a very little idea of what most careers actually entail. I chose my undergraduate degree based on the fact that it allows me to explore my academic interests more, putting aside a career for the minute.
Addressing your two concerns:
1. You'll be working for 40 or more years of your life - whether you study for five or seven years will mean very little in the long term. Many Monash MBBS students also take the optional research year during their degree, meaning that they're actually studying for six years before they graduate. I don't think that the time taken should deter you from either pathway.
2. Well, this is something more personal and you'll probably have to weigh it up for yourself. I'm in a position where I'll pretty much be alone nonetheless, so it wasn't a factor I had to include in my decision-making. I'd say that you'd definitely make new friends at university regardless, however.
Some extra insight:
The main reason for pursing undergraduate medicine is the security of knowing that you've already earned your place in a medical degree. We all know how competitive it is both at undergraduate and graduate level and so if you are 80% sure this is what you want to do and can manage to get a foot in the door, then go for it. I know that I definitely feel more comfortable with that sense of security. The graduate entry pathway is somewhat a risk which may not work out initially. This is something you'll need to be prepared for.
The main reason for pursuing graduate medicine is the ability to explore the area more and make a more informed decision prior to committing to medicine. You're not locked in, so you are able to ponder a variety of career paths and you may find something other than medicine that interests you more. That being said, if you're in the Monash MBBS degree and you change your mind, there's nothing stopping you from transferring. It's probably a little bit more messy that way though.
Anyway, I'm probably not in the best position to help you out but I hope this has been able to shed some light nonetheless. Good luck making your decision.