Whereas the quality of answers you give during the test is important, you should not forget that the test has a time limit; and you should therefore be capable of answering all the questions within the time allocated.
There is no maximum word count. The minimum word limit is 150 words for task 1 and 250 words for task 2. It's recommended however, that you don't exceed 180 and 280 words for task 1 and task 2, respectively.
A Lexical resource is the vocabulary used in the writing. Vocabulary doesn't mean using fancy/ complicated words. It means displaying a variety of synonyms i.e. not repeating the words on-and-on.
It's not. Each one of those criteria is worth 25% of your grade, so nothing is more important than something else.
Using idioms is another malpractice that destroys your essay's originality. What does not come naturally, should not be used in writing, just to make it sound fancy. Scattering idioms throughout your essay only makes it culturally unsound. Remember your essay should represent you, your ideas, opinions and your culture.