Ok, so I'll try and go through each question individually.
1/2/3. The essay seems to give students a lot of trouble each year. I'll answer your 2nd and 3rd questions first as they are easier.
The essay is not a descriptive essay. It is about expressing your point of view on the topic - and then arguing why your point of view is correct. So you should pass judgement indeed. At every point in the essay, you should be thinking about how each sentence contributes to your analysis. Imagine the essay to be like a longer version of an 8-mark "evaluate" question.
Structure and expression are only important in that they should make the essay readable. You won't gain or lose marks for it theoretically, but if it's filled with grammatical mistakes and poor vocab then the examiner's view of the whole essay will likely change. Your intro should begin by introducing the topic and also the thing you want to talk about - so a brief introduction to the UN, or states, or whatever it may be for AOS1. Then you want to express your point of view on the topic and introduce your paragraph sub-topics - like an English essay. Conclusion is not as important, but basically you want to restate your point and sum up your most important reasons for the point of view that you are arguing for throughout the essay. The examiner's report (
http://www.vcaa.vic.edu.au/Documents/exams/politics/intstudies_assessrep_11.pdf) contains examples of introductions and advice on the essay.
So on question 1, the essay section of the exam/SAC is as I said above, about arguing your point of view. You want to include both analysis and detailed examples and evidence. You also want to look at the "other side" - opposing arguments - and then think about why your point of view is superior. The actual essay-writing bit is quite similar to English I guess. If we take the topic given for AOS 1 in last year's exam:
"'States hold considerably more power and influence than non-state actors'. Discuss", you might choose to agree/disagree/partially agree with the topic. Each paragraph would look at maybe one type of power and how states/NSAs hold it, or you could spend a paragraph on states, one on UN, one on TNCs etc. I'm sorry I can't be more specific here, if you have an essay you have written then I can look at it and tell you if you're on the right track.
4. This has some truth in it but you should use the topic you're most familiar and comfortable with. People tend to not do AOS1 in the exam as there are so many possible topics and also the AOS is quite broad. Personally, I chose to do AOS4 because the Unit 4 topics are "issues" rather than being descriptive topics, which makes them more suited towards analysis in an essay. Also, Unit 4 will be more fresh in your memory and the topics are much easier to predict. In fact, I got the exact topic in the exam that I had done a practice essay for! But ultimately it comes down to choice and you can do well no matter which topic you choose.
Hope this helps! Feel free to PM me if you have any more questions or want me to take a look at your essay.