Just keep on practising on probability and remember for topic sentences don't make things complicated. Just make it simple but with enough depth to make them sound decent enough.
They just compared your results to the number of candidates who took the same exam as you on the medstart, which i'm assuming not many people took it. Thus it resulted in that percentile that you acquired. So don't worry too much.
Technically you did not fail. 46/80 is a pass. Just try to work harder leading up to the trials and of course the HSC and i'm sure you will get what you deserve.
Hmm so far i don't really like productions of materials (too much content).. acidic environment is ok. Lol the calculations for AE is not that bad actually, its a build up of what you did during prelim.