Even with stuff where knowledge varies and there's no true answer (like customs, which may vary from area to area, from family to family), there is a point where somebody's authority on their own culture shouldn't be questioned.

Maybe I'm misunderstanding you. But it sounds to me like you're saying that ultimately a "native" or "authentic" person of a given culture can't be wrong. Take customs. Since customs vary, let's say your hypothetical Welsh man lives in a place where the custom is X; are you really in a position to say, no, you're an *authentic* Finn, and therefore the custom is Y, and you can't be questioned on that?

This is actually a serious issue in my own country (the US) right now. An awful lot of people feel like there is only one way to be American, and we can't question that. And people who aren't "authentically" American (i.e., people who aren't white descendents of northern Europeans who have been here for at least three generations, in this narrative) are wrong, wrong, wrong,and have no right to say that we're not a Christian nation.

I don't disagree with you that you are a greater authority on your culture than the Finnish-Canadian teen or an author with no apparent research skills--or even an author with fantastic research skills!! It was just that last bit that sit really uneasily with me.