I think Texas Mist and Passionflower are on to something. Sounds like they have had experiences not unlike Susan or myself....
My theory of what happened is based on what I have read, seen and heard so far on this case as well as personal experiences in the church and with a psychopath. Any licensed clinicians with additional experience with psychopaths, narcissists and abuse please feel free to chime in.
Background: I think around the time Susan had her first baby things she only barely noticed in Josh's personality started becoming more distinct. The control and power issues were now at full force. (Someone on the earlier news report said Susan and Josh seemed to be doing well and were happy when they first moved to the area 5 years ago)
One of the first things an abuser will do is take the woman away from her support system (family in Washington) He has family in Utah. Obviously they did not move there because he had this wonderful, high paying career waiting for him. What he didn't expect is that Susan was strong and well liked and would develop a support system in her new community. It must have driven him wild to see how competent and happy she could find herself, especially without him being the reason for her happiness. After all she was merely an extension of himself as were "his" children.
Over the last couple of months: The power and control issues were getting worse and worse. Susan had made a couple of very smart moves. She had gotten a decent job. Josh had been telling her for some time that if she left, she would never be able to support herself. He probably said "Look at me, I can't even handle a full time job. Do you think a judge is ever going to give you enough support that you could live off it?" The psychopath always underestimates his victim. They may have started out with marital counseling but more than likely the counselor (probably from LDS Family Services) realized after a few interviews, it was an abuse issue not a marital issue. That means that Susan was probably in counseling with a private counselor as well as her bishop. Psychopaths never subcomb to treatment. After all they are smarter and wiser than any professional counselor so Josh would never see a counselor or admit the abuse. Susan was probably put on either anti-depressants, anti-anxiety or sleeping pills or some combination of these. This would give Josh the opportunity to start telling people that Susan "wasn't right" or that she was "unstable". He would love to believe that but no one else was really buying into it. So things started getting worse and worse for Josh. She had to be stopped.
Currently: I think that Susan and Josh had an enormous fight or "power struggle" Saturday night or Sunday morning before church. Josh said he wasn't going to church, Susan went. Her friend Kriisi (sp?) said she didn't see Susan at church but saw her when walking home. I bet Susan was in the bishop's office telling him what had just happened. You DO see your best friends during church at least some time during the 3 hours you attend. Josh knowing his time was about out, made a plan. While Susan was at church, he took Susan's entire bottle of sleeping pills and crushed them, planning to put them in her food at dinner, then leaving with the boys for sledding to come home to find that Susan had killed herself with an overdose of sleeping pills. His plan backfired because as usual Susan messed with the plan. Instead Susan invited someone to come over after church for an early dinner. This drove Josh nuts as he had already crushed all the pills. What if Susan saw all of her pills missing???? So Josh went along with the plan. Very interesting how he didn't serve a casserole but make individual "pancakes" and scrambled eggs. I'll bet you anything that you can not taste medication in eggs or perhaps juice. Then he has to keep checking on her during dinner. This is the point where he starts getting schizoid because of the change of plans. He tells her friend that he's taking the boys sledding to show he's going to be out of the house with the boys for a while hoping that will cover his butte and get her to leave. Susan starts feeling poorly and needs to lie down. The friend leaves and he takes the boys out for a while to let his poor wife die in bed, figuring when he gets home he can put the bottle next to her bed. Only instead, when he left Susan got up out of bed, making it to the living room collapses on the floor where she proceeds to foam at the mouth or vomit. When he comes home he finds her like this. I think he's not the "good dad" as people make him out to be. Psychopaths are too lazy to be good parents. They are controlling parents. Josh would have put the kids either back in the car right away (oldest one up front as he can talk) or put them in a room by themselves while he put Susan in the van and started to clean up. He probably got all that together by 12:30. The oldest child must have known that Susan was in the back of the van, though and probably asked his dad some good questions. (that child and DNA are going to be Susan's saving grace) I think since Josh told reporters he was south and then west of SLC that Josh actually went just the opposite way, either north or east. I also think he pulled off on the side of the road and the oldest child saw "woods" or "trees" in the head lights. Not a forest, not a desert, not a heavy snowstorm but "woods/trees". Meanwhile Josh came around to the back of the van and took Susan's body out and either (must still be an active suicide plan) put her in a large body of water near a bridge or even worst put her in a sewer hole. I do think he's smart and just playing dumb. But I don't think he's brilliant and he's certainly no where near as brilliant as he thinks he is.
Also, I agree with the lip thing. It's almost a smirk of contempt for LE and the press. When he says I just want to find her, I hear him saying "and you won't find her". Listen and watch that interview by his car again. Seems like he's just evil and taunting the press.
Hey hey hey hey hey... There aint nothing wrong with extra coushin for the pushin... Us big girls need love too.... And some of us are like a fine wine, we get better with age.... 
