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.
It's not only a problem of language, it's also a problem of culture. To put it bluntly: Western People (US, EU) don't think in the same way as Eastern people do. This is the main reason why it's seemingly difficult to accurately translate the word "Aiki". Japanese is a language that's very context based (high-context) whereas English and other European languages are a lot more explicit: if we say "a", we mean "a" and not "b" or "c" or "d", depending on which context the word "a" occurs in.
If we, as Westeners, try to interpret O'Sensei's words, we will do so within a Western framework, ie. any interpretation will be subject to cultural bias. We will unconsciously project our values and ideas in our interpretation. The only people who would have been able to correctly interpret O'Sensei's texts are those who were close to him when he wrote those texts, because they lived in the same context as him.