As far as PD and what she does or does not know... I have a feeling that she suspects a lot, but KNOWS very little for sure. And, like another mother we all know too well, is more than happy in her denial.
I had a theory about the arm incident, that made me think this. I think that PD's version of the arm story is correct, from her point of view. She stated she was in the other room and she "heard" him fall up the stairs, but did not see it. Iirc, the sister was also present. But, I believe it is possible that JD faked that incident. I think Ayla's arm was broken earlier that day and he had to cover it somehow, bc how else to explain it? So, he pretended to fall on the stairs, knowing that his mother would overhear. I think this is why PD did not think it was that big of a deal when it happened, bc it is hard to fake a fall that would result in that type of injury, and so the "crash" was probably not sufficient in her mind to actually break a toddler's arm. She still seemed surprised during the interview that it had resulted in that type of injury... But, she is his mother and deep down knows what he is capable of. So, in that way, again like that other mother, is complicit by not speaking up and moving past her denial.
I think the same is true of the actual disappearance. I think she has been fed lies, and readily accepts them as truth, disregarding the evidence, bc she does not want to deal with the possibility that her son harmed her granddaughter.
Taking this theory further, it is possible that he has duped the others in this same way...
This is all just my own ponderings, but I find it hard to believe that three or more people would cover for any one person's action unless there is a heavy dose of denial involved.