Hey, I will hopefully also be doing Aero/Sci next year as well!! I'll also be relying on SEAS to get me in 
With the science bachelor, seeing as I have to have one major in maths, can the other be relativity (general & special) and quantum mechanics? This stuff really interests me and I'm keen to learn it if it is possible
Also, seeing as I meet the requirement from modeling techniques (Adv) and multivariable calculus (adv) in first year I was wondering if anyone can tell me how difficult this is compared to the non-advanced version and the extra work load and if it is even worth doing? 
With the double degree, as you said, one of your two science major must be in maths. You are free to choose the other two, and judging from the units you listed (relativity/QM), you'll probably be looking at a major in
physics. Do note that around 15 of my friends-aero/science, all wanted or was thinking to major in physics before starting second year. At the end of second year, only 2 of them decided to continue with their major in physics. Of course you may be the minority but I just want to give you a warning that the general consensus of uni physics is shit

As for doing the adv. version of MTH1035-modelling techniques and MTH2015- multivariable calc, I personally don't think it's worth it as in the adv. version, all you do is cover all of content that the normal unit covers, and a couple of extra topics and proofs etc.
The reason that I don't see it as beneficial are:
1. In the adv. version, you sit an exam that is slightly harder than the normal version, and you compete against a slightly stronger cohort. (scaling might take effect)
2. Following from above, doing the adv. version may lead to a slightly lower mark compared to the normal version.
3. You can crash the extra tutorial held for the adv. version of the unit. (Adv. students has to attend an hour (or two? I can't remember) of tutorials per week in addition to their normal two hours tutorial)
4. For someone that is really interested in maths, I think it would be better for them to choose what topics of maths they like to learn up on themselves rather than learning the extra stuff in the adv. unit which might not be to the liking of some people. In uni, if you like a particular topic, you should be the one taking the initiative to read up on the unit, contact the relevant lecturers who's research is in that field (You don't need to be enrolled in any of their units. Many of them are happy to talk) etc.etc.