1. Because English has become a lingua franca used throughout the world, learning it will help nearly anyone communicate when abroad or when meeting foreigners.
3. It's not impossible for adults, but very few manage it. Children younger than thirteen or so can usually do it. I have a cousin who immigrated from Russia to California at age thirteen. Forty years later, he still speaks English with a pronounced Russian accent.
5. Many varieties of English are so distant from the forms taught in school that they will be difficult for foreigners to understand. Some British and American accents are practically incomprehensible even to other Britons or Americans.
7. Yes. Being immersed in English will be tiring but you will learn much faster.
Student or Learner