I don't see how this can be so, Michael.
You say to separate the art from the manuals. This, in my view, cannot be done, for the simple reason that the manuals are the only connection we have to the art. The two are inextricably linked and there is no living tradition stretching that far back that is remotely verifiable.
I absolutely agree that there are an immense number of ways to describe a technique, limited only by the language being used. Equally so, there are significant similarities between the German and Italian traditions - but, as Matt has already pointed out, there are substantial differences. The lack of the meisterhau in Fiore, for example, considering that this is a man who, in his own words, learned from diverse masters, including Germans, during a 50-year-long career.
Now, back to the Japanese images above. The top guard position - a variation of hasso no kamae- is used to defend against cuts from above, but also to deliver a kesagiri, or diagonal cut from above.
Most interesting, it is a key guard for performing kobu-itai, which literally means that attack and defence are the same. You apply it just as your opponent has committed to his cut, to use everything together, posture, balance, technique, cutting angle. The principle is called settsuke, everything as one with total commitment.
Sounds like the defend/attack principle of the German school? So is it German? No, it's not. So, is it Ochs? Yes, in principle it is, but in cultural context, it is something entirely independent. It looks like Ochs, it defends like Ochs.
I'm perhaps being too simplistic here and example driven, but hopefully it demonstrates that manuals can say very similar things with absolutely no relation to one another.
That, of course, is not saying that some manuals perhaps directly cross-refer to one-another.
"Something horrible is happening inside of me and I don't know why. My nightly bloodlust has overflown into my days. I feel lethal, on the verge of frenzy. I think my mask of sanity is about to slip ..." Patrick Bateman.