1. Can I invent statistics to make my task 2 essays more persuasive?
I don't recommend it. I think it will look 'fake' and it won't help your score.
2. How can teachers help students to build their topic vocabulary?
I find that the best way to help my students is by using relevant articles or online videos. I try to find good texts, and I pull out the useful language. My ebook is a good shortcut for self-study, but extensive reading and listening is the key to building students' vocabulary knowledge.
3. What does "went on" mean in this lesson?
If I say "most of my money goes on rent", it means that I spend most of my money on rent, so "40% went on" just means "40% was spent on".
4. Is it normal that my speaking examiner was drinking coffee?
No, that isn't normal. While I usually tell students to ignore what the examiner does (e.g. checking his/her watch or making notes), I think in this case your examiner's behaviour was wrong. Maybe you should report the issue to your test centre.
5. If my score is lower the second time, can I use my first test score?
Yes, I think you can.
6. For the "describe a person" question, can I describe myself?
No. This question always asks you to describe someone else. You'll lose marks if you describe yourself.