I think it is a very real possibility that a metaphysical force we can not yet fully comprehend results in animation and the presence of a "spirit," as it were.

Of course, I have no "proof" of this. . .

Speaking as a "hard" (logical) atheist and anti-theist. Unless you have a faulty (theist or mystical) view of what the "spirit" is, you need none.

it necessitates an external arbiter to decide what is and isn't consciousness and moral agency.

Though I'm not a Rothbardian, I don't think it does. For example, does it take an external arbiter to decide who is and who isn't a human being? And do you consider yourself moral in any instance or case whatsoever.

If you know that you are, you don't need anyone external to confirm that you are.

But yeah, whatever one's stance. The state should stay out of it.

I'm with you on that. No previous violation of rights can make conscription (slavery) moral.