Wow, fascinating interviews. Thanks, Mark. I'll read them carefully but one point I noticed already was this quote:
Ishii Sensei: Well, at that time he was already retired and he didn't have a regular class. I was only 15 years old. I went to Hombu dojo many times, where there were many teachers teaching regular classes. I expected to see O'Sensei at the dojo. His home was just next to Hombu dojo, and I expected him to come out of his room to teach us. But he didn't come to the dojo often. After a few months I learned that it was only for Yamaguchi Sensei's and Kisshomaru Sensei's classes that he came to the dojo to join us. This means that O'Sensei was not interested in other teachers training.
Ishii Sensei concludes in the final sentence (and with the perception of a teenager) that O Sensei was not interested in other teachers. A powerful counter-argument would be that an experienced teacher like O Sensei would want to join the classes of the teachers who still needed support or supervision.