1. I don't know the actual numbers, but Extension generally scales higher. However, unless you do extension 2, one of your units of advanced will definitely count towards your UAI. And as always, there's no point choosing a subject that scales well if you're going to suck at it
2. There's comprehension in paper one for advanced, but it's very different to the SC comprehension - it's not multiple choice, for one thing. There's also creative writing, and an essay. The other paper is all essays. Extension is a two hour exam with an essay and a creative writing task
3. If you want good marks, don't even think about it. Memorising an essay will get you nowhere, because there's no guarentee it will actually answer the question you're being asked, and even if it does (very slim chance, by the way), the markers can usually tell, and you'll be marked down for it.
4. Advanced have to read at least one Shakespeare. I've never heard of a class who do more than one, though. The prescribed texts for each module of each course next year can be found here:
http://boardofstudies.nsw.edu.au/syllabus_hsc/pdf_doc/hsc_english_poster_09-12.pdf
I don't really find that the extension texts are any more interesting than the advanced ones, but the way we look at them does tend to be.