I do find it interesting that the group consensus falls on the side of etiquette and non-physical cultural aspects are necessary, but the particular clothing is not.
Once you get to the question of whether a specific
etiquette is required then I think that things get much cloudier.
The etiquette in Iwama is different than than in Tokyo, in many places I've trained in Japan there's barely any reiho at all. If we're requiring a specific
type of reiho then one could make the argument that there's only one "correct" reiho.
I think that trying to make the argument that a specific
reiho is required is going to be very difficult - and then you get to the point of where the crossover is.
A number of the direct students of Morihei Ueshiba told me flatly that it wasn't important if people didn't want to bow, for example. I know that there are some dojo where the entire practice of bowing has been eliminated - is what they're doing no longer Aikido?
I think that if you ask people "should etiquette be required?", then most will say yes. Once you get into the details I think that most will say "no".