Alright, what I think I know is:
Viscera is required for magic.
The viscera need not be from the kin of the one being magicked.
The Coppersmith is summoned with bloodied cloths.
The Coppersmith grants wishes (for lack of a better word) in exchange for the cloths.
The Crab Guardian grants additional years of immortality in exchange for the cloths.
What I assume from all of that:
The Coppersmith doesn't necessarily enjoy harming people, because we start the game asking for viscera by itself, with no particular wish in mind. With no wish, the Coppersmith wouldn't gain a bloodied cloth, so I have to assume the Coppersmith intends to grant wishes without taking viscera from the family. So why bother with taking the viscera from kin at all? My guess is that it's part of the mythos/religion to keep the peace and prevent people from trying to solve all of their problems with magic. People would probably think long and hard about asking for something when the price is the death of a family member. Plus, what would stop them from murdering strangers to get the viscera to pay for the wish?
What I want to know:
Are the Artisans capable of magic before they contract with the Crab Guardian?
Is there really another Coppersmith, or is this one's reputation so bad (Don't eat many children!) that the suicidal girl assumes there must be another one? EDIT: Another comment mentioned an impostor, so it looks like I missed something.
Do the other Artisans do the same type of magic?
Is the Coppersmith altruistic, or only doing this to maintain immortality?