Third portrait
為野尻君
宇土囗雄
For Nojiri-kun
(signed) Uto ~o (Hideo, Shigeo, some given name ending in "o")
Regarding #4, I cannot be 100% sure the photo is the same person as the Seigō Nakano I linked to in Wikipedia. But the name is the same, and the features are so close I think it would be a pretty interesting coincidence if they were two separate people. I also wondered about the LA link. From the quick search I did on Seigō Nakano, he apparently traveled to Germany in 1937, and passed through Honolulu on his want back to Japan in February 1938. It is possible his return trip also made a stop in Los Angeles. Being a notable figure, and still at peace with America (albeit an uncomfortable one), it is not unusual he would have been worthy of news attention in Little Tokyo at the time.
Anyway, I cannot be completely sure, so we shouldn't discount the possibility that this is another Seigō Nakano.
These pictures are great. I was just reading up on Ninomiya's studio and the efforts to archive the photos found in the building where his studio was located. Are you part of that effort? If not, the people archiving the photos might be able to help also.