Can we use "you" to talk about people in general?
是否可以用you来代替people？

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...".
在口语考试中，when you go on holiday, you have the chance to relax这样的说法没有问题。但在写作考试里，我更倾向改成when people go on holiday。

In the speaking test, what should I do if I understand the question but I have no idea what to say in my answer?
在口语考试中，如果我听懂了问题但不知道说什么的话该怎么办？

You could use this formula: "Well, I don't really know much about... because..., but I suppose..." In other words, you admit that you don't really know what to say, you explain why, then you invent an answer anyway.你可以使用这个句型：Well, I don't really know much about... because..., but I suppose...。
换句话说，你承认自己不知道说些什么，并解释原因，然后你再虚构一个答案。

Is it ok if my pronunciation is a mix of British and American English?
如果发音混合了英国口音和美国口音，会有问题吗？

Yes, that's fine. Just focus on speaking clearly.
没问题。重点在于表述清楚。

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。