1) Should I apply for a re-mark?
From the comments below this lesson, it seems that scores most often remain the same when students ask for a re-mark. I would only recommend applying for one if you are really confident that you did well, and you only need a 0.5 band improvement in speaking or writing.
2) How much of the writing score is given for task 1 and task 2?
You get one third of the score for task 1, and two thirds for task 2.
3) Do I need to include all of the data in my task 1 essay?
No, the question will always ask you to select key points. There is usually too much data for a 150-word essay. Also, your ability to select key data will impress the examiner.
4) Should I put a future sentence in my task 2 conclusion?
No, you don't need to do that (although you can if it's relevant). I usually just repeat or summarise my ideas in one sentence. It's better to spend more time on the main paragraphs than on a long conclusion.
5) Do the reading and listening tests become more difficult in each section?
The listening test does; section 1 is easy and section 4 is the most difficult. However, the same is not true of the reading test; sometimes the first section is the hardest.