Tomoo - thanks for the added details - and your hypotheses are very plausible.
I did receive some communication from a senior Roppokai practitioner regarding the technique that Shioda reportedly received from Horikawa:
The technique is an Aiki Otoshi where you lift you grabbed arm a little up before dropping the guy straight down. According to Okamoto, Kodo told Shioda "I'm going to throw you down now, ready?", right before dropping him. When Shioda was dropped he responded "Ah got it!".