So many things on this case make no sense- but then again, murder doesn't make sense either, so maybe it's just that a sane person can't understand an insane one.
But I'm just putting this out there- from the beginning, it's been odd to me that it went down the way it did. I could see him losing it and killing SW in the middle of a 4am argument, I guess, although he has no history of violence that we know of, and it really makes no sense. People divorce all the time, very few kill their spouses to end the marriage. I can't honestly see SW killing her babies, but to be honest, I can't see him doing it either. They just always looked so happy together, all of them. Normal, with a bit of bickering or nagging, sure, but that's married parenting life. But if it was premeditated or he killed the girls earlier, why did he leave the crime scene such an obvious mess? Why didn't he have things thought out better, for coverup? If he killed the girls earlier, and then waited for SW to get home, he would have had time to plan how to conceal it, probably thought through all the details- like calling to postpone/cancel her dr appointment, or set the stage so it looked like she left him or at least they were burglarized. And yet, he didn't. And if he killed SW first, then why kill the girls at all? He could have just loaded everyone in the vehicle, called in to work taking a day off, propped SW with a pillow and blanket and told the girls (in their carseats, unable to reach mommy) that she was napping because she got in late, then drove somewhere and dumped her, and claimed she walked off, drowned, whatever. He did none of that. It's like the whole thing happened and he was just in shock.
His behavior reminded me so much of when one of my kids does something they shouldn't, and another one covers up for them. Like they're playing with grandma's glass dog figurines and one gets dropped and shatters, and they all lie and say "we weren't, it just fell" because no one wants to see their sibling they love get sent to time out.
I honestly wonder- what if when SW's flight was delayed, the AP came over because CW thought SW wasn't going to get home until the next morning? What if SW walked in on them, and that's why the sheets were ripped off the bed? What if they all three got into an "emotional conversation"- and then while SW and CW were arguing, because he does love her, and unlike what he told AP, he wasn't leaving her, which made AP mad seeing him not standing up for her and kicking SW out, AP was the one that went down the hall and strangled the little girls, to punish CW for not choosing her, and CW and SW saw THAT on the monitor and ran down there? (I think it's possible he saw a woman strangling one of the girls and the girl dying, although he didn't realize she was blue until he made it into the room, and in his mind, in grief-shock state, he just combined the two images into one).
Stay with me- but if that's the case, then I could see SW and AP getting into a fight, obviously. If CW loves AP, or heaven forbid, she is also pregnant (wouldn't be the first time we've read of men getting a wife and mistress preggo at the same time), and he intervened to protect AP from SW (which explains why he wasn't trying to resuscitate the girls, because he was dealing with the other drama), and then killed SW, it might explain the last minute, poorly thought out attempt at cover up. And the weird way he totally confessed to all of it and then changed his story slightly to say he didn't kill the girls- if he was trying to cover up for AP. And if he knew the hands around the little girls' necks were a female's and therefore smaller than his own hands (which would show up in the coroner's report), then he decided to blame their deaths on SW, who was already dead, and leave AP completely out of it, so she'd stay out of prison. He wouldn't be the first person to take the blame on a crime someone else did, as a cover up. Especially if he felt guilt that the whole scenario was his fault, for having the affair to begin with.
I know it's random, but it doesn't seem that far fetched to me. He could have sent AP out the backdoor, through the construction site (a lot of people having affairs park a bit away and sneak in so not to be seen by neighbors), back to her car, and tried to clean up. And then, because he felt disloyal to SW, he put on the shirt she gave him, as a way to try to connect to her and make it up to her for killing her. And it would explain why he seemed so detached and calm- if he was trying to just hold it together for the sake of the AP and protecting her, and was in shock at how everything had gone down.
And on another random note, I wonder if he put the girls' bodies into the tanks to protect them from getting dug up and drug around by wild animals, and wanted to do the same with SW, but SW wouldn't fit. Like a burial at sea of sorts, but last minute and panicking. The whole thing just seems really erratic and spontaneous. AP is the wild card. But her presence in the house that morning helps make some of the weird "why" moments make sense.