I think you D is honestly doomed for the rest of this year on the group project front. If you have talked to the teacher and he did not change his approach at all, he is likely going to have to learn the hard way that he is expecting more than most middle schoolers can give. You are not obligated to improve his performance -- it sounds to me like he will get some knocks from parents of more average kids without you pushing this further. But it may take him a while to revise his assignments and plans based on those lessons.
Probably the best you can hope for her from this experience is the knowledge that a grade lower than an A is not the end of the world... As long as she learned what she was supposed to for her portion of the project and turned it in, I would leave it at that. It is sixth grade; you likely have two more years left at this school at a minimum. I say keep your gunpowder dry for bigger issues.
