Yes, Val, good points. But I also think you're right that when it comes to informants being dishonest or misleading there's really no way to prepare for that. I think that the "ethical" fieldworker also has to utilize a well-honed BS-detector (an essential part of a folklorist's arsenal), and if they feel like they may be getting led down the garden path they should cross-check the facts with others, such as the hypothetical "Elders."
I've experienced, with a number of groups, instances when I've gotten in the middle of an internal power struggle over who is the cultural authority. In those cases I've tried to back off, because those issues can go on for generations and it's not my place, necessarily, to mediate. You know, I just try to be as honest and thoughtful as possible, and trust my instincts.