I would expect someone who held the view “B” to be somewhat agnostic on something like the origin of the universe, and I suppose most are. As something outside of our observable space and time and not subject to the scientific method,
I am indeed "somewhat" agnostic on the origin of the universe. None of the standard explanations, whether from science or from religion, really make much intuitive sense to me. However, I disagree that the question is necessarily "not subject to the scientific method". Just off the top of my head I can see where future discoveries should be able to help distinguish between the several theories that have the universe coming from a prior universe in some sense, and the ones that have the universe popping spontaneously "out of nowhere".
our explanations of it are all attempts at educated speculation. In light of this, any theory which satisfies some logical considerations (the cosmological argument, etc.) should be considered at least possible, and the theory of a Creator certainly does that. So if one believes in “B,” I would expect one to say that, if the origin of the cosmos was not a chief concern of science, then perhaps there are other ways in which we can obtain knowledge in this area - especially considering the fact that science already affirms certain truths, such as the “sensory evidence” principle, which are not verifiable by science.
But as I argued earlier, just because the idea of a Creator isn't scientifically verifiable doesn't mean it's an axiom like the axioms of science are. It has to actually be necessary for rational thought or discourse to begin in order for it to be an axiom. You've never shown that it's logically necessary to assume a person of some kind behind the origin of the universe in order to begin to rationally think or talk about it. You do have to assume the fundamental reliability of sense evidence, as well as the principle of non-contradiction, to begin to think or talk about science.
Isn’t it entirely possible that the existence of a Creator could be shown to be as valid a theory as naturalism, despite the fact that it does not chiefly reside in the domain of science?
Because of the above, I say you can't call it a "valid" theory until you can show that assuming a creator-person is behind it all is necessary to begin to think about the creation of the universe.
As for B vs. A, I (and I assume most scientifically-minded types) believe the answer is B, but we note that throughout history, as more & more aspects of the world became known more & more intimately, the result has always been A: The theories that were not scientifically verifiable have always turned out to be not true. Strictly speaking A isn't correct, but it has always been the way to bet.