Sociology is a VERY vague subject. You can never go into a paper saying 'I did great!' because there never is one correct answer. You'll be taught the entire course and it'll be up to you how you fill out the answers, these can be up to 25mark long questions so it's not easy at all. Olevel sociology is nothing like Alevel sociology so it won't help you out one bit,
If you can revise Econ on a daily basis then you can surely do it, especially if you're good at diagrams. You seriously can't afford to lack behind in economics. Olevel economics will only take you so far in Alevels. The Alevel course basically starts off from scratch, only in a more particular and precise direction. AS economics, rather than Olevel economics, will be the basis of the subject.
As for Law, it easily gets one of the worst grades. A friend of mine got 3 A*s and a B in Law, despite saying it was the easiest of the bunch. I would highly recommend you not to. It's really not worth the hassle.