You need a 4 year degree to teach. Whether TESOL or any other program is beneficial is questionable. But you have to have a degree. Find a job, the company will sponsor you for your visa. So if you are planning on coming now, you can forget it.
1- No guarantee. That's like saying if you and your friend move to San Francisco can both of you get a job in the same industry. Maybe so. Well, I assuming that your friend will also want to work. If not, your friend will be limited as to how long he/she can stay in Japan, but free to stay where he/she likes.
2- You should know Japanese to get around. Which may tie into your work. Say you get a job where the company is sending you out to different places to teach. If you don't speak some Japanese, getting around can be very difficult. Most places don't require any Japanese skill to teach though. But you'll need some basics in order to live here.
Yes, it is SO useful to have a degree in something like English literature when teaching a beginning student to say This is a pen, That is a pen, These are pens, What are those, What is this. So incredibly useful. I can't begin to count the number of teachers I hired to teach English. The best one had a degree in Physics and has since moved on to become a doctor. The number of teachers hired with degrees in English? Zero. General intelligence, the ability to teach and a good personality are the major traits desired. You need a degree in English literature about as much as a fish needs a bicycle.