So in this interpretation, yuu 幽 refers to the world of the mind, hence Ellis' reference to the subconscious. Would this also have been Ueshiba's interpretation?
No, in this interpretation, yuu
refers to things not visible or tangible, the workings of the mind being but one example. To be clear, the translation as "subconscious" is not Ellis', I believe, but rather the translators of the Aikido Journal article he was referencing.
As for Ueshiba's interpretation, that's dicey, because he didn't need a translation. He just used the word, along with all of the various nuances and shades of meaning the word has. To pick one is to limit the idea, which is why I've always tried to give it multiple glosses. My main intention was merely to show that the two varying translations are not as discrepant as you at first may have believed.