I read a lot of mysteries. I can't remember offhand which detective says it can be more informative if someone lies or refuses to answer than if they tell the truth. [It's a bad paraphrase. I think it's Nero Wolfe.]
Anyway, I think if a competent interviewer- someone who wants to find Natalee- makes the attempt, the Kalpoes could provide good leads even inadvertently. If there is remorse for the part they've played or a desire to clear their name, all the better.
The Kalpoe brothers should be able to tell us how they all three came about telling the fabricated story about dropping Natalee off at the Holiday Inn.
Did Joran force them to lie for him? Was Paul Van Der Sloot involved in the fabricated coverup?
Yes, you're right. They can provide valuable information to us. Good job.
sw