That was all in battle scenes, or in the narrative leading up to them. Those teeth-edged weapons would probably not work very well for head-removal. One needs a truly sharp edge for that and flint or obsidian or the like is the answer in a pre-metal civilization.
I also have warriors carrying stone axes in the series (i.e. the three Malvern novels and the new ‘God of Rain’) and those would certainly do the job — if they were the right sort. The heavy ones carried by some men are more like maces, meant for hammering at the enemy rather than cutting. But lighter ones, again with heads of quartz, flint, volcanic glass, would behead handily.
Now that flint-edged sword or ax is utilized ‘in the field,’ but would it be used for the ritual sacrifices in the Kohari temple? Those people do make some use of copper, primarily for ornaments, and hold the metal sacred to their goddess, Mihasa, the Sun Bird. Her image in hammered copper stands above her altar. I could see a copper ax being crafted for the neck-severing blow in their ceremonies. Is there one? Irrelevant at this point but we might find out someday.
Or not. I’ll probably never include such a ceremony in one of my novels. I don’t even see revisiting the temple, even though I have two projected novels to go in the new ‘Mora’ trilogy. There are more important questions, such as ‘how do they make their beer?’ or ‘how are dogs treated?’ (as a food item, I am afraid). Things of this sort have more to do with who a people are and how they live. They are far more relevant to the story.
And also we might ask, ‘why are these people headhunters?’ That might be the most important question of all!