My theory, with massive gaps:
All my opinion only.
I think Ron went to work, and Misty was at the home with the children. I think most of the day played out as reported. I think at some point Misty killed Haleigh. I don't know why, maybe she wet the bed, maybe she wouldn't go to sleep, maybe she didn't feel good and was cranky. Maybe she got up and caught Misty doing something and Misty panicked. I also don't know if Tommy was there, or if he came in later.
I think Misty, probably with Tommy's help, disposed of the body. I then think Misty, probably with Tommy's help, came up with a story to tell Ron. I don't know what that story could be, but IMO it had elements of Ron being at fault for what happened, whatever that was being something Ron could get in big trouble with the law for, and also Misty being attacked in some way.
I don't think Ron is a sociopath, but I do think his number one priority is to protect himself. So I think whatever Ron was told made him feel guilty towards Misty, and maybe made him think, at least for a little while, he had a chance of getting Haleigh back. I just don't think Ron had it in him to put his daughter before himself. I also think Ron's ego played a big part in him believing the lies he was told.
So, at the end of the day, Ron believes he is protecting himself, which IMO is in character. Misty is not covering for anyone but herself, which is in character. Tommy is covering a little for Misty, but mostly for himself, which is in character. Joe had nothing to do with any of it, isn't covering for anyone, knows nothing, so was very easy to point fingers at. Hank doesn't know, but suspects his kids are involved, and that's why he's acted like he has. And all this is what Ron was withholding from LE, and what he used to buy his plea deal, why he needed immunity, and what he can testify to. None of this provides enough evidence, though, to charge anyone.
End of theory.