A couple of thoughts:
1) Iwama kumitachi has it's roots in Kashima Shinto-ryu (not Shin-ryu). Several of the initial kata are "copies," although very much changed in spacing, timing, cutting targets, etc.
2) Inaba sensei is NOT an expert of Kashima Shin-ryu. You can do a search of Karl Friday's (menkyo kaiden in the system) posts on e-budo and I think Aikido Journal re this. Inaba learned the first level sword kata and leavened them heavily with aikido. He may be expert at what he does, and it may seem like a small thing to outsiders, but to initiates in the systems, what he does is radically different from the ryu itself. It is true, tho', that Yamaguchi sensei was quite influenced by Inaba's sword - and it could clearly be seen in his aikido technique.
3) Nishio-sensei, I believe, learned a lot of his weaponry at the dojo of Matsuo Kempu (who taught Araki-ryu gunyo kogusoku and Shindo Muso-ryu jo, as well as iai.) As the poster above wrote, he definitely made the weapon's his "own," in remarkable fashion.
4) No one has been able to trace the origins of Osensei's jo kata. (Solo forms are very rare in traditional ryu.) It is my belief that the kata is Ueshiba's form of misogi (purification ritual) mixed with elements of weaponry he learned elsewhere. (The circular movements at the end of his kata with the spiral upwards is very often seen in Shinto ritual).