When you say to go to Japan for graduate study, you should be admitted to a university.
Not all universities and tutors recruit graduate students. Even in universities with postgraduate system, not all tutors are willing to take postgraduate students. Most of the five beauties you mentioned are private universities. It seems that Japanese universities should have a postgraduate system.
Tama University of Fine Arts has a postgraduate system.
These universities openly recruit international students to study university courses. Don't worry about this. The specific requirements depend on the recruitment requirements of universities.
Without the postgraduate system, you can directly apply for colleges, or go to a language school and then take the college entrance examination.
The last question cannot be answered. I specialize in the art and I am a layman, so I can't judge the difficulty. There are more than 40 international students studying in the Graduate School of Musashino University of Fine Arts.