I came upon your post and wonder what you might think about a couple ideas I have had in the back of my mind.
There are titles that identify a type of Martial Art, the main one I can think of being "Karate". Then there will be styles under that? Eg: Shotokan.
Would Karate be a system of martial art and the sub groups styles or would Karate be to broad and generic a term to be called a system?
I think Aikido could be called a Martial Art System and the various groups such as Iwama, Yoshinkan, as styles of Aikido.
This application of style is at a broad level compared to individual stylistic differences by instructors.
I think each of the recognized groups such as Iwama maintain the principles of Aikido but had a founder who decided there was at least one fundamental difference in how they would go about practicing or teaching Aikido from any of the other titled groups. Ki society focus more on teaching Ki for example. This difference would have to be more than a superficial one.
Instructors within that group may have personal style differences but have bought into whatever made the group identify themselves in a special way and will maintain the Aikido principles and whatever the groups sub principles are.
What can be confusing might be a Dojo where they say they belong to a group such as Iwama but their practice doesn't follow any of the direction given by its head eg: Saito Sensei.
At a personal level style reflects body shape, interest in aspects such as Koyunage techniques over pinning techniques.
Lastly, I wonder if there is a point where a style of Aikido no longer is Aikido because of the direction they go. Would competition mean its not Aikido because O'Sensei said there was no competition in "Aikido"? Perhaps its just in the interpretaion of what did he mean by competition.