1. What is the difference between bands 7.5 and 8 in writing and speaking?
The difference is small. In the writing test, you need a small improvement in one of the following four areas: 1) answering the question in a bit more depth 2) developing your ideas in a more organised way 3) using a wider range of vocabulary 4) making fewer mistakes. In the speaking test, my advice is to focus on adding detail to your answers in parts 2 and 3.
2. If the question is about 'working at home', is it ok to repeat those words?
Yes. It is inevitable that you will repeat some of the words from the question in both writing tasks. Try to add some variety where you can, but don't get stuck trying to think of five different ways to say the word "work" or "home"!
3. The examiner laughed at one of my answers. Is that a bad sign?
No, it's probably a good sign. Maybe you said something unexpected and the examiner liked it!
4. In writing task 1, can we join the introduction and overview together?
Yes, it's fine to put the introduction and overview together as one paragraph. Just make sure that your overview is good because it's an important part of your essay.
5. Can we use quotations in writing task 2 and the speaking test?
Personally I don't recommend including quotations because it's difficult to use them in a natural and relevant way. It usually seems like the student has 'forced' the quotation into his/her work, and this won't impress the examiner.