1) Is it a good idea to use headings to organise my essays?
No, you should never use headings in any of your IELTS essays. Don't include lists or bullet points either. All of these things are considered incorrect format.
2) Is it ok to write "I think" or "I like to say" in my task 2 essay?
Those phrases are a bit informal. I prefer to write "I believe" or "I would argue".
3) If I write that I completely agree, can I still write a paragraph explaining the other side of the argument?
Imagine the examiner's confusion if you say that you agree, but then give good reasons for the opposite opinion. If you want to include the other side of the argument, either write a paragraph explaining why you think it is wrong, or change to a 'partly agree' introduction.
4) Do I have to give my opinion in the main paragraphs as well as in the introduction and conclusion?
If you have stated your opinion clearly in the introduction, it will be obvious to the examiner that your main paragraphs are supporting that opinion. Your opinion should be clear throughout the essay (if you want to get a good score), but that doesn't mean that you need to write "in my opinion" before every idea. Have a look at some of my essays here on the site to see how I do it.
5) Which is correct: "A and B showed an increase" or "A and B showed increases"?
Both are fine. Try Googling "they showed an increase" and "they showed increases". You'll find examples of both.