First up, you almost certainly won't get into Oxbridge if your sole motivation is to make money. You have to have a passion for the subject(s) you want to study or you'll almost certainly be rejected at interview in favour of people who are.
To answer your questions:
1) Of the four subjects you've mentioned, I'd say that Economics at Cambridge/E & M at Oxford are the most competitive. Sciences are generally considered the least competitive, but you'd still need to be pretty damn awesome for Physics/ NatSci.
As far as A-levels go, it's about QUALITY not quantity. 5-6 A-levels do not put you at an advantage over people who are only doing 3-4. And yes, good grades are certainly a starting point, but they aren't everything, and it's certainly not as simple as "75% with all As get in compared to 10% of everyone else."
You can assume that everybody is applying with 3-4 predicted As, strong (generally straight A) AS results and a good handful of A*s and As at GCSE at the very least.
2) This makes absolutely no sense whatsoever. Are you saying you should apply for a less competitive course to maximise your chances? If so, if you don't have a passion in the subject you're doing, you won't get in even if the course is less competitive. So no, that won't work. Go for what you ENJOY and don't plan three years of your life on some theoretical employer at the end of it- particularly as your ambitions are so vague.
3) NO NO NO. Extra curriculars are all very nice, and make them want to want you, but if your academic potential isn't up to scratch, being a head boy isn't going to get you anywhere.