I spent 5 minutes of my precious exam time exuberantly reading into the scenario and the wording in questions 03 and 04. From my external independent study before hand I found that S47 can occur through skin abrasions, i.e grazes and such things that do not break all layers of the skin. I remembered seeing that minor wounds don't have to amount to a S20/S18 either - like a paper cut or a pin prick, however from my own experience using an air rifle (and consequently being shot by one in the back of the leg accidentally), the bleeding caused by it, even worded as 'bled lightly' in the scenario, must have sensibly amounted to wounding for a S20/S18, obviously such a close range pellet shot would have broken all layers of the skin so that is what I wrote about. Obviously brain damage can amount to the 'serious harm' needed for GBH, so no questions there.
In retrospect, I personally think AQA may have intended it to be S47, but also in retrospect I've looked at many legal definitions to see if the wording 'bled lightly' could have been anything other than S20/18, which it couldn't have been via how the real act intended it to be. So if AQA mark it for only S47 I would suggest contesting the validity of the mark scheme towards the specification and the law in real life. I think they might allow both S20/18 and S47, and give credit for both, however this wouldn't explain repeating the essay for question 04 on the brain damage, so I'm lost on the thinking and wording behind the exam paper for those two questions. Everything else was flawless.
Just to add, neither of the questions required students to apply causation as AQA would have asked if Ahmed had caused any injury to Julian, it stated in the question that Ahmed had already caused both the injury in the shooting and the brain damage and it just wanted you to discuss their criminal liability.