1) Is it necessary to follow the order? No, I've seen successful essays that deviate from the order.
2) Is it generally a good idea to follow the order of presentation? Yes, most applicants seem to answer the questions in order and reading essays in a different order can be distracting. I have found it better just to follow the given order unless a different order results in a significantly better essay. However, if you change the order it must be absolutely clear which question you are answering in each portion of your essay.