1) According to US News and World Report, Massachusetts Institute of Technology (MIT) is best. A large number of major schools have bachelors and/or masters programs so your options are great. If you want to narrow it down to a geographic area or a handful of schools, you can then go to US News to see how they stack up academically.
2) It’s not easy and you might find it’s not what you want to do and that is okay! You should do something you enjoy because you’ve got a lot of years to go after college. If you really enjoy it and are struggling, just keep going and work hard. You will struggle at times and there are definitely learning curves that you have to climb but eventually it’ll click and get easier.
3) They might not be. A lot of countries are going in to decommissioning and nuclear engineers are used primarily in reactor design so aren’t needed in decommissioning. You can still be in the nuclear industry with a chemical or mechanical (or any) engineering degree. I suggest chemical because you can do a focus in reactor/nuclear science and criticality, while having a broader range of prospects in a nuclear or non-nuclear future.
4) Countries with growing nuclear programs like China, India and the United Kingdom.
5) No, but you may find that the school you want to attend only offers a masters program and requires a chemical or mechanical bachelors.