Is it best to use "was" or "were" with "majority" and "proportion"?
It is normal in English to say "the majority of people were" but "the proportion of people was".
If the question was about an old friend and I spoke about an elderly friend, will I lose marks?
Hopefully the examiner won't penalise you for this mistake. The confusion is understandable.
For questions about where I live, should I talk about my hometown or where I live now?
It doesn't matter. You can talk about either.
If the question asks us to explain “problems (or causes) and solutions”, how many problems and solutions should we mention?
I'd go for two or three problems (or causes) and two or three suggested solutions.
The ebook talks about “consumer culture”. What does this mean?
It's a phrase that is often used to criticise societies in which people are encouraged to buy more and more things. Click here to read more.
If I want a band 7 in writing, is it true that 50% of the sentences in my essays need to be error-free?
The examiner will not calculate the percentage of sentences that contain mistakes, but the mark scheme does say that you need to "produce frequent error-free sentences" in order to get a band 7 for grammar.
Can I use the terms “x-axis” and “y-axis” for graphs in writing task 1?
Yes, but I tend not to bother with that.
Can I invent statistics in writing task 2 (e.g. 80% of children watch violent films)?
You can, but I tend not to. I'd just write something like: "a growing number of children watch violent films". Instead of inventing a statistic, you can probably find a better way to express the idea.