Separatist Peoples wrote:"Ritual sacrifice is not a punitive state-mandated punishment for a crime. It is not an execution. Even by that interpretation, it would not be banned."
My friend, you read much to literally. First define state-mandated. It is not defined within the international law cited. So, here we have a people within a sovereign nation who have condemned one to die. Often, these forms of sacrifice are as a payment to the gods to bring some favor. In essence, it is a payment for the group's sins. Said in another fashion: it is paying for all the group's crimes. As such it is the "sponsored and orchestrated death of an individual as their punishment for committing a crime". Show me one person who has not committed a sin (which is the equivalent of a crime in many primitive cultures). One could then argue that putting a person to death in this fashion is clearly an execution. However, it would run afoul of provision 4 where there is no trial let alone a separate sentencing portion of the trial.
As you can see, it is not very difficult to argue ritual murder or sacrifice is against international law as it currently stands. I'm afraid this does put some pretty damaging holes in your current theory...