There is another theory that I have kicked around a bit. What if it was the dad, and it happened before he went to work? Would DB cover for him? I think as hard as it is to think about, she might. She constantly looks at him out of the corner of her eyes, I find that strange. She has a good home with him, and she seems to be a person who takes a lot of guilt on herself. Could she have possibly thought, "If, I had done this or that, or been here or there, this would not have happened, and so I am going to support him, and go along with the story?"
In some ways she is so open and honest, that I believe that she might not be lying. But if she did flunk the LD test, it could have been because she knows what happened, and is covering.
A motive for either of them lying for the other could be the custody issue of the other two boys. In his case especially, if the mother lost custody and people speak of him as "ruthless" in getting custody of his son, perhaps DB would feel badly for him for that reason, as she too has custody of her son from a prior relationship.
I frankly don't know what to think anymore...a bunch of theories and nothing to really prove any of them, one way or the other.