I seem to have a biological imperative to define things. Ignatius, I like what you've done here:
In physical and mechanistic terms, which, as martial artists/hobbyists, we would want to concern ourselves with, ki is primarily motive forces - i.e. that which makes us/them move, or not move (as the case maybe).
But, I think as your need for the parenthetical there makes clear, it needs a tweak.
I think forces are the key thing, not motion, so how about ki = the likelihood that a body will impart a particular force?
This is a definition that fits rivers, seas, rocks, as people. Gravity is the ki of heaven because of the fact that because of it, any mass is 100% likely to be imaprted with a force equal to its mass times the gravitational constant. The ground has the ki of earth because (within reason) it is certain to impart an upward force exactly equal to any force put on it. And, humans have the capacity to do lots of movements, but when we formulate the intent (not just the plan but the real physical intent), our body makes arrangements for particular forces to be transmitted to particular points, causing the movement. How we do that is another interesting question.
But I think we can all see from our aikido practice that a body that is about to do a certain movement behaves differently than a body in a similar pose but just standing there. Isn't that the essence of the "magic" we feel when a technique goes right? We did the right things when our partner was in just the right state of intent?
(ps motion gives a body a form of ki too, under this definition. A body with a certain amount of momentum in one direction has a high likelihood of exerting a known force in that direction-- it just needs someone to hit for that to go from "intent" to fully manifested force)