I am not so sure we can glibly say that something like the Hiden Mokuroku was arbitrary and fluid. From what little I know of various branches including "aiki budo" era aikido I see a remarkable consistency of technical content.
Maybe not arbitrary in content, but Sokaku's art definitely seemed to be something of a "work in progress" and regularly changed and developed over the years.
While possibly unrelated, but I had completely forgotten about this conversation, but 9 or 10 years ago I was having a discussion with my iai sensei about his childhood and what not and asking about his father who ran a small jujutsu dojo out in the sticks up here in Akita as well as being very active in sumo as well. At one point I asked what his father had studied and he said "Takeda-ryu". I asked if this was connected to Takeda Sokaku and he basically said that it was what everyone today calls Daito-ryu, and commented that even the name "Daito-ryu" wasn't really used in that area of Tohoku and no one called it that and only started coming up later. Unfortunately my sensei's father died young and my sensei himself had moved to Tokyo in his later teens to study karate and didn't return until he was in his 30's so I'm not sure he even knows all that much about details of his father's training, although a couple of years ago he did show me an exercise his father had them do as a kid instead of shiko (for those curious, basically the exercise is you drop into a very low horse stance and then bounce yourself, while staying "in stance", forward, then back to center, backward, back to center, right, back to center, left, back to center, repeat. At first it just looks like you are jumping, but now I suspect there was more intended than meets the eye).