I will try to answer the questions you have asked:
1. Yes, there is an active scene for masters in Japan, and the depth is quite good in the master ranks, especially as you go to the older groups (60-70) some of those masters are at the top of their respective age groups on the world scene.
2. I see the Nishitetsu team train every day, and most of the runners wear the same tracksuits and sweats, but when training, they wear different gear. I think Nishitetsu sponsorship is not tied to one shoe company, because I see the runners wear different brands, with Mizuno shoes, appearing to be the most popular.
3. Other than Mizuno and Asics, the next most popular brand, at least in Fukuoka, seems to be Nike. Few athletes wear Adidas.
4. I`m not sure about media coverage for those foreign events, but I assume that they cover Boston, New York and Chicago, especially if there are Japanese runners taking part.
Questions 5 and 6 - sorry I don`t have the answers to those questions. The ultra runners in Fukuoka have no contact with the Nishitetsu runners, who concentrate on speed.
Ghost in Fukuoka, Japan