A lot of universities have requirements regarding A-Levels.
Most require 3 A-Levels (the full A-Level - AS AND A2), some may require 2, but most require 3, and all require at least 2.
These are built into the conditions of points-based universities too. A lot of universities will require 300 points from at least 2 A-Levels. So, if you got 200 points from A2 (2 Bs at A-Level) and 2 Bs at AS (50 points each), you'd meet the requirement.
Be sure to clear it with universities first though.
I'd advise choosing carefully between the two, but both are respectable subjects.
Probably not. If you intend on doing it, I'd recommend you do it outside of school (on a gap year maybe), as it will mess up your A2 studies.
GCSEs generally don't matter in all, and an AB over an AA won't be the be all and end all, universities generally care about predicted and AS grades more than anything.