Peter A Goldsbury
I have a copy of Roy Suenaka's book (published by Tuttle in 1997). It contains an illuminating discussion of the split, but does not contain a copy of Abe Tadashi's response.
Regarding Tadashi Abe response to Tohei's letter (which I haven't read), I wildly speculate that Abe and Yamada being relatives could have influenced somewhat the tone of the response.
Tadashi Abe had, before of Tohei split, critiqued harshly (it is said) aikido's evolution during the years he spent in France.