Balance is arguably an alignment onto its self, it will resist any opposing force that threatens its opposite. Harmony is a bit of a stretch as it implies peace, where in fact the fundamental reality of good and evil both existing is their antagonism for each other. If good and evil are light and dark then those of the alignment of balance are servants of the gray, they maintain both, attempting to insure that the world does not fall to destruction in stagnation or disintegration, intolerable placidity or infinite horror.

I agree that a true neutral is fundamentally without concern for good and evil, law, order, and chaos. It is of course applied to wild natural creatures because they do not have these concepts. A thinking being that lives purely for its self though is almost invariably a source of evil, but consideration for others is essentially the root of all good. Hence a sapient would be almost utterly impossible to exist as true neutral without actually doing so by effort (or perhaps more accurately application of a common sense reasoning of keeping your damn nose out of the affairs of primal forces to what ever extent is possible), which would more likely make them a creature of balance, if only with an inward focus.

A creature could also be labled as neutral if their agenda does not agree significantly with ither the course of good, evil, chaos, or order. Such a being however does probably have an agenda and is likely in their own eyes right, where right is both an arguable form of good and order, even if their definition contains acts of destruction and chaos.

Given that there is no extra alignment of balance in the D&D world, as defined here, we must consider neutral to be a generalization of a larger set of sub groups, just as to what ever lesser degree the other alignments are.