My problem with "spirit" is that "ki" has spiritual, mental, and physical (tangible) meanings and all three are important in aikido. Spirit hits 2/3 at best, completely missing out the physical part.
Shrug. "Aiki" as a concept in Japanese martial arts well-predates aikido. There doesn't seem to be any doubt Ueshiba had some things in mind during his teaching, but "aiki" as a description of a type of waza is well established. Still, some of the parsing strikes me as reminiscent of arguments about what the pleats in a hakama "mean" - an odd question since that form of trousers far predates the principles typically bandied about. Sometimes, it's just pants.