1. Is there a difference between "Do you agree or disagree?" and "To what extent do you agree or disagree?"
Not really. I would just answer both questions in the same way.
2. My teacher told my to stick to one side of the argument for 'opinion' essays and give both sides for 'discussion' essays. Is that right?
There are 2 ways to answer an 'opinion' question (click here), but you might find it easier to follow your teacher's advice (to avoid confusion).
3. If I only rely on your website and ignore all the other books, is a band 7 achievable?
Yes, many students have demonstrated this. If you read too many different books, the differing advice will probably confuse you. I tell my students to use the official Cambridge test books along with the advice on my website. You don't really need anything else, apart from maybe a teacher to check some of your work.
4. Why do I keep getting 6.5 in writing?
I was speaking to a student about this problem yesterday. It turned out that he had never written a plan before starting writing task 2. If you don't plan your task 2 essay, it's likely to lack ideas and organisation.
5. In the listening test, should I write "gas works" or "gasworks", and "state rooms" or "staterooms"?
Don't worry too much about this. Both answers would usually be accepted.