No, if the argument is circular, it's not actually demonstrating anything because it is attempting to prove the premise it begins with. You can't prove something with itself, otherwise you are merely stating something as a tautology, which is not an argument at all.
Not effectively, that would be begging the question, and then you're making a prescriptive distinction of morality. I don't care if there is a scientific way to determine morality, there are far more effective ways to determine moral systems that don't rely on arbitrary prescriptions which are often oppressive. I've already said before that the harm principle advocated by classical Liberals is by far the best starting point for a moral system. I don't actually think that moral truth, good or evil exist at all; the concept of morality is merely an extension of the innate human desire to create systems of normative behavior that are a requisite part of forming complex social structures. For that reason I support the use of debate and dialogue to produce ethical systems that do the least damage and allow us to function as a society. Religious systems that begin with the preconception that their way is inherently right by divine ordination are not conductive to the effective running of a nation that respects the individual and doesn't accept arbitrary oppression.
They're still a product of physical processes and thus are within reach of materialism. Subjectivity has no true meaning beyond that of the individual, you can not determine the truth of any experience or existence beyond your own through subjective means. Of course, we're social creatures that have to interact with others if we want to live so we have to bring in subjective things at times, but we should always be skeptical of conclusions drawn from subjective experiences. It's fun to share ideas and experiences too.