Re: Origin of ushiro ryote and ryokata dori
My take is derived fro Budo Renshu...
I think the origin of rear attacks in aikido is related to a balanced curriculum, with the emphasis on creating a rear-oriented attack for practice. As for ryokata, ryote and the other multiple grab options, you are also talking about a generalized attack attack to managed. I feel the attacks are designed to create a sterile canvas to work on, not necessarily a "practical" attack. Budo Renshu has several photos as well as a selection that speaks specifically about training to defend all angles of attack, even the rear.
Whether those earlier attacks had origin as more "practical", I get the impression for Budo Renshu that we should use [generic] attacks as tools to train the aiki body.