I did GCSE music and was absolutely crap at composing and I still managed to come out with a 7. I used musescore for my composition which may be easier for you because you are composing on a score rather than however garageband/bandlab works.
I was of a grade 5 standard on clarinet when I did my performance and I came out with a really good mark for it. On a different note one my grade 6 pieces was from suite no. 4 of Bach's cello solos.
I don't know anything about the other subjects you are asking about.