1. This is one of the many instances of one teacher giving recommendations and passing them off as requirements, I'm afraid. No, you don't have to mention more than one text and you certainly wouldn't get "bonus marks" for mentioning all of them. If anything, most assessors would probably dock points since you aren't going
beyond the texts, you're just limiting yourself to multiple ones.
I suppose I could see the logic of reading a brief synopsis of the other texts since the prompt has to relate to all four texts, and so it
can give you some insight into the kinds of angles the assessors are expecting.
The external examples are usually a lot more important to the marks because that's what you'll be using for your more unique and expansive discussion. In fact for expository essays, I'd suggest just referencing the text once in your first paragraph and then everything from then on is purely based on your other sources.
HOWEVER, your teacher will have their own recommendations for context, so trust their guidance for SACs.
2. Certain external resources are used quite often, but that doesn't mean they're used effectively. Something like mental illness is a very common example, but if half the people using it are doing so in a 'people who have mental illnesses have different realities, therefore sometimes people can have different realities' capacity, then doing some more sophisticated analysis can still help you stand out.
There's a reason these areas are so well worn, though; it's because they're often the best and easiest examples to convey. So it's more about the degree to which you're expounding upon the evidence than what specific evidence you're considering.
3. There are very few official requirements for the oral because it's just an in-school assessment. So you'll need to check with your classroom teacher about any additional stipulations they've put in place. Usually the only rule is that you have to find a news article from the Australian media since September or October last year. You might find it helpful to
make your issue local by showing how climate change affects us on a nation-wide or community-based level, but otherwise it can be a global or international topic provided your school is okay with it