How many would act morally if the law did not exist?
Morality doesnt have to be a arbitrary, transforming quagmire. It depends on what its based upon, what axioms you are operating from.
If morality is about human suffering, then slavery is clearly not moral. If morality is about doing unto others as you would have them do to you, then slavery again is wrong. It depends on what morality is based on, then you can operate from that to determine whats moral in a non-arbitrary, transforming way.
So now youre going to shift the burden of “transformability” of morality to those axioms, fair enough. But I dont think those are arbitrary (correct me on using that term if thats not what you meant by “complete transformability”) either, I think we determine the basis for morality the same way we do for everything else. What makes sense as a basis? What is effective as a basis? Those questions have answers that are not arbitrary, at least not in any sense that science or other things we don’t think of as arbitrary wouldnt also be.
So maybe a person doesnt have any basis or axioms for their morality, but I dont think its accurate to call that an arbitrary or completely transformable morality. I would describe that as not having morality, as what is moral/immoral has no meaning anymore. It would just be whatever the person feels like doing whether it makes sense or not. As soon as a person wants to make sense, they have rules to follow, a non-arbitrary basis.