It is pretty well documented that O Sensei threw dan ranks around like they were hot potatoes. I think this had a lot more to do with his feelings about rank than with his definition of aikido.
Fair enough, but not conclusive - and it ignores most of what he actually said when defining things.
All the technical curricula you mentioned above have irimi, tenkan, ikkyo, nikkyo, sankyo, kotegaeshi, koshinage, etc. I have trained with students of the Saotome, Nishio, Yamada, Chiba, Tohei, Homma, Tomiki, and Hombu curricula, and they all used nearly identical terminology for essentially the same set of techniques. We might not all agree on what is good aikido, but virtually all of us do seem to agree, according to a technique-based definition, on what is aikido and what is not.
So if that definition is insufficient, (1) what's wrong with it, and (2) what is your alternative definition?
As I said, some of them would state categorically that what you are doing isn't Aikido. Moriteru Ueshiba has stated categorically that what some of them are doing isn't Aikido. So it's hardly that simple, despite similar sounding names.
Ueshiba himself stated that Aikido was a principle based art - so did Takeda. It follows that anyone following those principles (leaving aside for the moment the question of whether or not any particular MMA guy is doing so) can be said to be doing Aikido.
"Aiki-do" = "The Way of Aiki" = anybody practicing the principle of Aiki could be said to be doing "Aikido". "Aikido" itself, as a term, isn't exclusive to Ueshiba's art anyway, so it can get a little tricky.
Of course, this could open up a can of worms too...
I wrote some of this up last year in this blog post