With Diane, I think she's more a bumbling and confused player than a deliberate manipulator. It all hinges on what she said in the HoH room after Adria won. Did she actually say Will should be put on the block or did she merely say "one of them?" I remember that she did tell Adria that she wanted Marvin gone, so it seems like she was calling attention to the other members of her team just to take the heat off herself. But she didn't want to seriously target any of them. Maybe she underestimated how much this would "paranoy" the twins.
If the twins do out Diane this week, I don't think it will help them as much as they're hoping it will. Karen and Nik suspected the worst about the twins, and this week they had all those suspicions confirmed. I don't believe K&Nik would take the twins at their word even if they are telling the truth.