I'm not sure if this horse is dead or not, but bouncing around the Interwebtubes, I came across
this article (PDF format) by Shishida Fumiaki, in English.
I leave it to the reader to form their own conclusions based on it. However, just to add some context from the side of Yagyu Shinkage-ryu:
Gejo Kosaburo is quite the mysterious figure. Ostensibly, he was (according to Akabane Tatsuo) an ultra-high level master of Shinkage-ryu. However, little is known about him in the Yagyu Shinkage-ryu mainline. Some question whether he was really a student of Yagyu Toshichika (Genshu) at all. It seems obvious, at least from Shishida's research, that he attended the Hekiyokan, the Yagyu Dojo in Wakamatsu, in Tokyo. Here's the problem. The Hekiyokan opened in 1913, when Gejo was 48. Until that time, Yagyu Genshu was in Nagoya. Genshu returned to Nagoya in 1922, at the age of 77. Genshu's son Toshinaga (Gencho) continued teaching in Wakamatsu until 1930, when he also returned to Nagoya to care for his ailing father. So, if we assume that Gejo studied Shinkage-ryu purely under Genshu (after losing a match to him) in Tokyo, his training would consist largely of 9 years, from 1913 to 1922, ages 48 to 57. Even considering an accelerated pace of learning, and physical genius on Gejo's part, I personally have difficulty believing he could become the great, highly licensed master Akabane portrays him as. Which is not to say he wasn't good, and didn't advance in the curriculum.
Anyway, we come to the interesting question of Otsubo Shiho. On the Arakido site and in interviews with Kajitsuka Yasushi, Otsubo is said to have been a student of first Genshu-, and later Gencho-sensei. Akabane says, rather, that Otsubo was a deshi of Gejo. Given that Otsubo's age, and the fact that Genshu-sensei left Tokyo in 1922, and Gencho-sensei left Tokyo in 1930, I'm inclined to believe Akabane on this, with the caveat that Otsubo likely learned from Gencho-sensei when Gencho-sensei gave lectures in Tokyo just before and following the war.
On the other hand, this presents some problems. We have Shishida saying Otsubo was present when Gejo taught Ueshiba. If so, Otsubo was describing childhood memories. Even more confusing, is here it seems that Otsubo had first hand knowledge of Gejo teaching Ueshiba YSR, and yet according to Kisshomaru, sometime in the 1950s or so, expressed surprise at Ueshiba's movement and guessed that he must have learned YSR.
Given that Gencho-sensei's son and the previous soke, Yagyu Nobuharu was alive during all this time and may have been able to answer many of these questions of who trained where and when, it's a shame that Shishida never thought to interview him.
Finally, I personally find Akabane's research, mentioned in the article, rather problematic. One reason being his curious tendency (common to many Japanese researchers) to couch opinions as fact and to present ostensible facts with no sourcing whatsoever. Another is his tendency to form opinions (then presented as facts) based on a kind of comparative analysis of kata, which is always extremely dicey, IMO. I've read the article (in Japanese) that Shishida cites here regarding Gejo bringing aikido tai-sabaki influence into his YSR, and this is just such a case. I find Akabane's arguments unconvincing, and shallow in understanding of the kata of YSR. Akabane's own experience in YSR is from an offshoot he says is in the line of Otsubo Shiho (apparently not Muto Masao's line), which he abandoned about three years ago to join the Shunpukan, a dojo founded by Kanbe Kinshichi (miswritten as "Kobe" in Shishida's article), who was a student of Genshu-sensei.