I think the biggest mystery in this case is why Darin stood by Darlie when he has to know she did it. It's just so obvious and it makes him look guilty even though he isn't. The 911 tape clearly shows Darlie explaining to him what happened when he first came downstairs and found her on the phone. She even promises him she didn't do it. They couldn't have staged that. Besides, if you are going to conspire to kill your kids you can do it in a lot easier, less violent way.
They fought that night and her maniacal-diet pill anger was really aimed at him, IMO. She even threw him under the bus in one of her jailhouse interviews years ago when someone asked her if she thought Darin was involved and she paused dramatically before saying no.