1) Is the listening test marked by a computer?
No, this is a completely false rumour. Read the start of this page.
2) In the Cambridge IELTS books, what do the answers in brackets mean?
They are optional answers. You don't need the words in brackets, but your answer will still be correct if you put them.
3) A teacher on a different website said that we should write "it is disagreed" instead of "I disagree". Is this correct?
No, that's really bad advice. "It is disagreed" means "most people disagree", so it doesn't answer the question "Do you agree or disagree?". Remember, the rules for IELTS are not the same as those for 'real' academic writing at university. It's fine to use the word "I" in the IELTS test.
4) My teacher (in the USA) told me to write 4 sentences for the introduction to task 2. Do you think examiners here might prefer that approach?
No. Examiners are trained in the same way in every country. It's a waste of time writing a 4-sentence introduction. Just write 2 sentences.
5) Can you explain the structure of this sentence that you wrote?: "Just pop round to number 10 any time after 5 pm."
It's an imperative sentence. Click here for a full explanation.