Paul, what are the restrictions on the order of words as they are translated to English? E.g. Is it the way of harmonious spirit, or the spiritual way of harmony, or the way to join with spirit, or the way to a life of harmony?
At this point I should clarify that I only have about a year of Japanese knowledge. However, I can hopefully steer you in the right direction.
There is actually no grammatical structure implied in the word "Aikido". That is, there's no genitive or possessive case involved. Japanese compounds don't quite work like that. It's just a string of concepts with no linking grammar.
I do think it's reasonable to view it as "Aiki-Do". That is, aiki arguably forms a subset. Aikijutsu, aikido, etc.
So put another way: "The Way of Aiki". Where Aiki is a concept of matching one's ki or fitting with one's "energy". As for "energy" or "ki", I do defend the "spirit" translation as reasonably correct; the English term "spirit" matches reasonably closely in terms of usage. Like, it can be ethereal or purely psychological, for instance. It can refer to "energy" ("Do it with spirit!") or to "attitude" ("in good spirits").
So the real question is: what is "aiki"?