Re: "Hidden in Plain Sight" - Shinkage ryu menkyo
Have you seen the book entitled 『会津藩教育考』? It is the source of much of the information about Kurokochi Dengoro Kanenori that Kono Yoshinori used in his Aiki News article, which Ellis in turn used for the chapter on Kanenori's activities in the Aizu domain and his supposed relationship with Takeda Sokaku. The book is part of a series about Japanese history published by Tokyo University (in the 1930s, I believe), but it contains far more information. I have not gone through the book specifically for information concerning the Yagyu Shinkage Ryu supposedly taught in the Aizu domain, but I think it will be there.
I think you would find the book of interest, not least for the interesting style of Japanese in which it is written.