1. Is it ok to use the words "I", "my" and "we" in writing task 2?
Yes, it's perfectly ok to use those words. You might be told to avoid them in university assignments, but the advice for IELTS is different. If the question asks for your opinion, you will probably need to write "I agree", "I believe" or "in my opinion".
2. Can we invent facts or figures in writing task 2?
I don't recommend inventing statistics or news stories. I always think that doing this makes writing seem 'false'. Just explain your own ideas and opinions, and give reasons and examples.
3. Can we use "you" to talk about people in general?
In the speaking test, it's fine to say something like "when you go on holiday, you have the chance to relax". But in a written essay, I would prefer to change this e.g. "when people go on holiday...".
4. Is it ok to use "firstly" and "secondly" without a "finally" idea?
Yes, that's fine.
5. Will I lose marks if I don't give both views in writing task 2?
It depends on the question. If the question asks you to discuss both views, you must do this. However, if the question asks whether you agree or disagree, you definitely don't need to mention both sides of the argument.