This is exactly the same problem when looking at texts like the gospel of John or other christian texts. It is only that generations of scholars have done their work so we know more about it and have a broad discussion about the different issues.
And every theologican learns hebrew, greek, latin. We study history. We do linguistics. ... We learn how to study the old texts.
When it comes to the thoughts of Ueshiba first we have no clear control text. Most people are not able to read or speak the original language, i.e. Japanese. The historical knowledge meagre. And so on. Most of us are just dilletantes.
So it is not methodological problem, but a problem of our scholarship. I think.
Well, good points, not that I disagree - I just think it can be an advantage when the inspired person is not the author himself, but things got sort of simplified, for lack of a better term, by a generation or two in between before someone wrote them down ....