Great post!

Would the choice to extrapolate the values of humans alone be an unjustified act of speciesism, or is it justified because humans are special in some way

I choose neither. Whatever we coherently want at a higher, extrapolated level, asking if that is justified is to attempt an open question type of argument. "Specialness" isn't a justification, understanding the meanings of "justified" in people's idiolects and extrapolated volitions would allow one to replace "justified" with its content and ask the question.

Setting aside the possibility of cohering people's unmuddled volitions, an unmuddled volition is a self-justified one.