I find commerce really interesting and I never did anything commerce related before, in fact I thought it was one of my weaker areas. If you intend to do actuarial without much commerce background then some units may be a bit iffy but that would be in first year i.e. accounting units hate them lol
You may be able to do Commerce with MD prereqs but it would restrict your Comm degree, you would probably only do 1 major but that's a guess. If you plan to do actuarial there is no way you'll be able to fit it in, you only get 3 breadth units.
Comparing actuarial honours to MD biomed is hard, they'll both be difficult no doubt, but it would depend on the person as to which is more difficult. You would want to maintain very high grades in both, perhaps not as high for actuarial but it would still need to be quite good. Difference is GAMSAT for MD and interview; however, you have the accreditation units for actuarial where you need to get a 73 ave between 2 subjects and there are quite a few modules of these that you need to satisfy to be accredited. Both equally difficult in some regards but it's hard to judge which is harder relative to the other, particularly since none of us done either lol But I don't think that's what you should make your decision on... You've probably heard this over and over but do what you WANT TO DO and WHAT YOU THINK YOU WILL ENJOY. I guarantee you that if you think Commerce will get you a secure job after 3 years and you say to yourself yep let's do that then you will regret it if at this stage, you're not sure if you want to do it - especially since you have med aspirations and you're probably fully capable of getting into PG med. It is easy for your grades to fall below par in uni especially in 1st year commerce units where a lot of stuff is relatively simple, I know a lot of people who barely passed core units and some who failed a few units. I know it's a tough decision, I was in a similar situation as you but I'd realised med wasn't for me by then. Go with your passion and heart, if you fall short then no matter there are other jobs out there and you will be in field that enjoy and if you're good enough there is nothing stopping you from earning big $$$ if that's what you're keen on.