Is it a ok to start task 2 essays with a question?
2. 在长作文中,是否可以用问题开头?

No, that would be more journalistic than academic style.
不可以。这样看起来更像是新闻,而非学术写作。

In writing task 2, is it better to write four paragraphs or five?
在长作文部分,写4段好还是写5段好?

There isn't really a 'best' way; it's possible to write a band 9 essay with four, five, or even six paragraphs. My preference is for four paragraphs because this encourages you to go into more detail in each of the two main body paragraphs. More detail and depth = a better essay!
并不存在最佳选择。4段、5段或6段的作文都有可能拿到9分。我个人更喜欢4段,因为这能促使你在文章主体的2个段落里进行更深入的讨论。更多的细节+更多的深度=更好的文章。

Is it a problem if I mixed British and American spellings?
如果混用英式和美式拼写,会有什么问题吗?

Not really. This isn't something that is likely to change your score.
不太会。这些不太可能对你的分数造成影响。

If I only put my opinion in the conclusion, will I lose marks?
如果我只在总结中写了我的观点,会丢分吗?

Possibly, yes. If you want a high score, your opinion should be clear "throughout" the essay (if the question asks for it). State your opinion in the introduction, then explain it in the main body, and repeat or summarise it in the conclusion.
可能会。如果你想要高分,你的观点应该在整个文章中清晰体现(如果题目要求这样)。在开头陈述观点,在正文进行解释,然后在总结部分进行重复或概括。

Is it ok to use the words "I", "my" and "we" in writing task 2?
长作文中是否可以使用I、me和we?

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".
可以,完全没问题。你可能听说在大学作业中要避免出现这些词,但对于雅思来说情况并不相同。如果问题是问你的观点,你可能需要用到I agree,I believe或in my opinion。

Can we invent facts or figures in writing task 2?
长作文中是否可以使用编造的事实或数据?

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.
我不建议编造数据或故事。我总是认为这样会让文章看起来很假。只要解释你自己的想法和观点,并给出原因和例子就可以了。

Can we use quotations in writing task 2 and the speaking test?
在长作文和口语考试中,是否可以引用名人名言?

Personally I don't recommend including quotations because it's difficult to use them in a natural and relevant way. It usually seems like the student has 'forced' the quotation into his/her work, and this won't impress the examiner.
我个人不推荐使用名人名言。因为你很难找到和题目相关又不生硬的名言。经常能看到考生在文章中强行引用名言,这并不能给考官带来好印象。

How do examiners know whether I'm using 'memorised language'?
考官如何知道我在背诵答案?

All of the language in your brain is memorised, so I should explain this properly. The examiner would be impressed by a memorised phrase like "greenhouse gas emissions contribute to global warming", if the question asked you to explain an environmental problem. However, examiners are not impressed by the kind of phrase that students think they can use for any question e.g. "This is a controversial issue that has led to heated debate among people on both sides". Can you see the difference?
大脑里的语言都是背好了的,因此我会用适当的方式解释这个问题。如果考题是让你解释环境问题,“greenhouse gas emissions contribute to global warming”这类事先背好的答案会让考官印象很好。但是,考生们认为有些短语可以用于任何问题,例如“This is a controversial issue that has led to heated debate among people on both sides”。考官对这类答案不会有好印象。你可能找出它们的区别吗?

Should we give essays a title?
应该给作文起标题吗?

No, your essays don't need titles, so don't waste time inventing one.
不。作文不需要标题,不要浪费时间在这上面。

Is it a good idea to use headings to organise my essays?
给作文段落起标题好吗?

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.
不好。在雅思作文中永远不要使用标题。也不要使用列表和编号列表。这些都被视为不正确的格式。

Is it ok to write "I think" or "I like to say" in my task 2 essay?
在长作文中可以用I think或I like to say吗?

Those phrases are a bit informal. I prefer to write "I believe" or "I would argue".
这些短语有些不够正式。我喜欢用I believe或I would argue。

Should I start new paragraphs by missing a line or with an indentation?
段落用空行区分还是开头空格区分?

Both ways are fine, but I think it's clearer if you miss a line.
都可以,但我觉得空行显得更清楚。