2) Impossible to answer, it varies every year.
4) Yes, possibly.
Sorry for the blunt answers, but I recommend declining the D grade in CT - that is exactly what I would do. Modulely speaking, you are not so strong, but besides that 4A's is not bad. I would dare say your GCSE's are above average, but A-levels - slightly below average, if we take into account the module grades. But you still have a chance if you are up for the challenge.
Ok, here is what I would do: apply, because trust me, you are not wasting a UCAS spot, do not mention about the full marks in modules - it's overrated, and do you best in your PS and interview. Make sure your teacher writes a good reference about you. Do well in your coming A2 exams.
And finally, just don't worry too much about it. With 4 A's, I wouldn't worry too much.