1. I followed your advice and used a 4-paragraph essay structure, so why didn't I get a higher score?
Good structure and organisation are important, but the expression of ideas is even more important. IELTS is a language test, so the key to a high score is to express relevant ideas using good language.
2. How many bands will I lose if I only write 130 words for task 1?
I can't give an exact answer to this. The examiner will give you a lower score for 'task achievement', but you might also lose marks for vocabulary - a shorter essay probably contains fewer examples of good language use.
3. Can I use 'double' like this: "The figure for X increased to nearly 20% in 1997, which almost doubled its initial rate in 2000."
The use of 'doubled' is ok, but I found the whole sentence a bit confusing, maybe because the years seem to be mixed up. Can anyone suggest a clearer way to write this sentence?
4. Can I use the word "skew" to describe numbers in task 1?
If you Google the word "skew" or "skewed", you'll see it used to describe numbers and data. However, it's only used in certain situations, and I've never used it in an IELTS essay. I wouldn't worry about this word if I were you.
5. Can I say "There were twice as many precipitation"?
No, you would need to write "There was twice as much precipitation" because 'precipitation' is an uncountable noun.