No. I assume you will be going to university after, so when looking for employment, they will look at your degree, rather than how many A Levels you have. It's definitely not worth the stress either. Better to have 3/4 high grades than 7 middle grades (assuming doing so many will affect your grades).
1) I think not. Just stick with double at most. If you wanted to go into maths, go for triple, if you think you can take it, and have your life set on maths.
2) A lot!
3) Lol, no. You really need to learn how to choose.