To me, Darin covering for Darlie doesn't make any more sense than Darlie covering for Darin. Why would he want her out of prison (especially in the early years) if he knew she'd killed two sons and that she could go back to her family life with Devon and him? Wouldn't he fear for his son's life, and for his own?
Also, what I previously said is not that Darlie was intentely covering for Darin, but rather that she didn't think it was him so she said ''no'' when they asked her repeatedly if her husband was the murderer. Tell me, if the police and prosecution didn't think that he could've been the killer, then why did they ask her this question over and over again?
Right now I'd qualify myself as a fence sitter. But I don't believe in the intruder theory... or at least not without the involvement of at least one of the two parents.
And, about the fact that Darin would've had to go through the utility room, garage, back yard, through the gate before coming back inside... I think he had other options:
1- He planted the knife at the entrance of the utility room and, while she was turning back to turn the lights on, ran past the nook and upstairs. There is a wall there and, if the timing was right for him, he could've done that without her noticing him. Darlie saw the man head towards the utility room. She turned around, hit the lights, then got to the utility room and found the knife there, which made her think the killer was hidden in the garage or had escaped through it.
2- Again, let's pretend Darin was the killer. He left the knife at the door of the utility room, went to the garage, closed the door (was it closed? I can't remember for sure), then out the window into the yard. He waited a few moments outside (he could probably see her move through the house with the kitchen light on, and also hear her screaming). The moment he saw her leave the room, he came back inside through the sliding glass doors in the Roman room. There was a partial print (in blood, if I'm not wrong...) found there so someone had to have used that door at some point. Also, in the pictures, we can see that the blinds were higher on the side of the entrance than on the other side. So anyways, let's say he gets back in the room through those doors, and Darlie is now at the bottom of the stairs screaming her lungs out for him to come down. The rest of this kind of puzzles me and this is why I prefer scenario number 1 over this one. She would have seen him downstairs and not getting down the stairs...