why single out this particular equilibrium and energy scale?

Because the domain of chemistry is broad enough to contain life as we know it, and also hypothesized forms of life on other planets, without being excessively inclusive.

I think you're missing my point, which is that I don't see how your definition excludes fire as a living thing.

I tried to answer it. The chemical species that are produced in fire are the result of equilibrium reactions http://en.wikipedia.org/wiki/Combustion . They are simple chemical species (with more complex species only being produced in small quantities; consistent with equilibrium). Especially, they are not nearly as complex as compared to the feedstock as living chemistry is.

I don't think it does. A human in vacuum is alive, for a short time. How do you distinguish between "virus in host cell" and "human in supporting environment"?

They are both part of living processes. The timescale for 'self-sustaining' does not need to be forever. It only needs to be for some finite time that is larger than what would be expected of matter rolling down the energy hill towards equilibrium.