I recommend spending a good hour searching this forum and reading previous posts. This will give you a really good and balanced ideas to your questions. There is lots of good information already here for people like you, and then there's also a lot of biased or misleading information too. Read between the lines.
But in my opinion, in answer to your questions.
a.) yes, no problem. And pick the A-levels you enjoy the most and will put effort into.
b.) Whilst cheap may sound good, you've got to be careful you are actually getting quality. Airlines will always like to recruit from the best schools first, and therefore those schools charge a premium to train there. Some say its worth paying, some say its a rip-off, purely a personal choice. You make your own risk/reward calculation. I suppose if you are an outstanding candidate at assessment, the airlines might not worry too much where you got your ATPL from, but you'd need to get the assessment in the first place!
c.) Aptitude tests massively vary from school to school, but in my opinion the maths tests are never more than GCSE standard. Some schools even let you use calculators. Of all the things to worry about becoming a pilot, this is not one of them. You either have the ability or you don't.
d.) I doubt it, some might even argue its an advantage!
e.) Of course, but you will also need lots of money and to talk about"life skills/experiences". Start preparing for both now.
Good luck!