Hello! I'm glad that I've finally found someone else who does Music 2 on this forum

I wouldn't call myself an expert at this topic, but I'm answering to the best of my knowledge!
a) The repertoire matters in the sense that it must be good enough as a standalone to get you into the top band - i.e.) it's not a good idea to "rely on" other pieces in your repertoire to get the marks. You should be confident that every single piece, even when assessed individually, should be enough to get you a strong result. This is why it's important to choose "good repertoire" - which is something that demonstrates your technical ability and highlights your strengths - e.g.) if you have a strong high register, you would choose a song that is mostly in the higher register, or has climactic moments in the high register. Being "sophisticated" matters in a sense that you shouldn't be choosing something that is too easy, or else you are not showing enough of your skill for the examiners to know that you have a very strong technical ability - which may limit your chances of getting in the top band. Ultimately, it's about finding a balance between choosing something that is sophisticated enough, with something that you can realistically prepare within the next 7 months or so!
b) It's really hard to define "Music of the Last 25 Years (Australian focus)" and identify specific features, because it's so diverse in nature (which is the exact problem that I'm having with my comp). So what I would suggest is to exaggerate and highlight any extended techniques in your piece. An example might be a glissando (I'm not very good with vocal techniques, sorry) or a note that doesn't belong to the key signature - you would emphasise that. Again, it's hard because there's not much we can do about it and the topic is so diverse in nature.
Best of luck for the year ahead and hope this helped!