I am glad to see someone newer than me posting on here, especially with an intelligent argument.
I think ultimately the easiest definition to prescribe would be on a conditional basis based on what item we were attempting to define, though you still end up with things that fall into both sides in some way or another. One example is food. Almost all foods have some part of them that is inherently natural, even twinkies probably have some sort of flour that makes up the cake. Or maybe it would be easiest to go with definition where something can be partially natural but not completely, like the dog breeds mentioned earlier, or even guinea pigs. Even rape, as horrible as it sounds, has components of it that are partially natural in some way. While these things may have been made entirely by human hands, they have part of them that exists outside of human intervention, and thus I don't think can be considered entirely unnatural.