1. Can I use the phrases "bottom out" and "level off" in writing task 1?
Personally I never use those phrases, but you could try Googling them if you want to see some examples.
2. Will my speaking score be lower if I miss one of the bullet points?
Maybe. You should always try to cover all of the points on the task card, but you can still get a high score if you speak well about the other points.3. How does the examiner know if language is copied or memorised?
Every word that you know is copied from somewhere and held in your memory. However, the phrases that examiners don't like are the ones that students learn because they can be used in any essay about any topic. For example, examiners are not impressed by the phrase "this is a controversial issue nowadays". On the other hand, you might have memorised the words "the greenhouse effect", but if you're writing about the environment, it's perfectly fine to use them.
4. Do you update the topics in your ebook?
No. The most common topics will always be the most common topics. The IELTS exam is over 20 years old, and they have always asked questions about topics like work, family, education and environment. I doubt that will change.