Re: Non-Aikido thoughts and considerations
If we really want to use a broad brush and define the founder of "aikido", then it would have to be Sokaku Takeda. There are no others.
Prior to Ueshiba, Takeda would have been learning koryu (as we now call it). It would not have been "aikido", nor would the art have been called that, nor would the art have dealt so heavily with "aiki" related techniques as we see in modern times.
So, no matter who taught Takeda, it wouldn't have been aikido in any shape.
Now, we jump to Sagawa's father. Takeda has been out and about and teaching. He's teaching Sagawa's father and we get this from Transparent Power:
The elder Sagawa, who sometimes had a fiery temper, would take what he learned from Takeda and try it out on strong and mean-looking construction workers he came across. He quickly realized that if you lacked the sort of aiki that Sokaku Takeda possessed, none of the techniques would work against a persistent opponent. So Sagawa's father said to Takeda, "I'm already so old, I think it would be better if you'd teach me Aiki instead of techniques."
Takeda was teaching aiki separate from techniques even at that time. Then who comes along? Morihei Ueshiba. He trains for a few years and then relocates to Ayabe.
What happens when Takeda shows up? According to Noriaki Inoue in Aikido Masters Prewar Students of Morihei Ueshiba:
Aiki is so astounding that even Deguchi saw its importance and suggested to Ueshiba that they add aiki to the name Daito ryu. Ueshiba is hesitant but Takeda seems to accept the suggestion and does change the name of his art. Even after the name change, Ueshiba still doesn't want to use the name aiki.
Takeda changes the name. I think it's rather funny (and perhaps Ueshiba saw the humor in it, too) that later, it was Ueshiba who gave the official okay to classifying and naming these "different" arts, aikido.
In a broad brush, Sokaku Takeda is the father of aikido. Ueshiba is not the founder or father. He made it highly desirable to learn by other martial artists. Kisshomaru Ueshiba and Koichi Tohei altered some things and made aikido famous and known worldwide.
In a broad brush, the way of aiki encompasses: Sokaku Takeda's own personal martial art, Takeda's Daito ryu students and their lineages, and of course, Morihei Ueshiba's own personal martial art and Ueshiba's students and their lineages.