I believe you are correct. These kinds of things were clearly important to Ueshiba, but the question at hand is not whether or not they are important, but whether or not they are essential to the definition of aikido. And nothing I've ever read from Ueshiba indicates that he believed they were.
But the thing is, you might not see him say specifically "the hakama and dogi, as well as clapping and bowing, are essential to Aikido." Because for this to be remarkable at all would require that he thought of them as distinct things at all, and not part of a larger gestalt of cultural transmission.
If he is quoted as saying the hakama is important at all, that really does tell you something.
In general I think the thing would be to look for cases where he tells students they need to attend to details, keep themselves clean, not be lazy, not be sloppy, stuff like that.