Why is it Japanese instructors seem to be so reluctant to use groups ?
This has not been my experience. I attended seminars in Chicago during the early 1980s when Toyoda sensei had a pretty small dojo. One of my teachers, Rod Kobayashi, was also there on some occassions. They encouraged us to practice in groups to protect ourselves from getting injured.
P.S. The decision about who is the best teacher for you should not be based on race. Basically, I feel that I can learn something from just about any high-ranking teacher, no matter where he's from. My personal preference is for someone with big skill and small ego.