My opinion only. It sounds convoluted but might have been possible.
Steve Powell was obsessed with Susan Cox as he related to a woman acquaintance numerous times between 2001 and 2009. He had "trophy" underwear of hers he had stolen. He had pictures he had taken of her without her knowledge. He was addicted to *advertiser censored*. I believe he sexually abused all of his children creating a psychological hold on them which may explain why all except one of them (at the ages of 25, 28, 33, and 34) were living with him in late 2010 according to the Washington State Voter Database.
Susan's disappearance was planned. His daughter said that he and Josh were on the phone more than usual before Susan disappeared. Once it was decided to murder Susan I think the sociopath, Steve Powell, decided he would have his way with her and planned how to do that. When Josh fixed dinner that day, Susan was probably drugged as part of Steve Powell's plan.
To drive from Puyallup to Salt Lake City would take a long day to get there (about 14 hours plus stops for gas, bathroom, and to eat), time to sleep, time to commit his crime, and a day to drive back. Three days. If my theory is correct, he probably flew there and back after commiting his crimes which could have taken as little as one day.
He could have taken the earliest possible flight on Dec 6th, rented a car, rented a room at a motel, ate, slept and driven close to Josh and Susan's home by 5 pm, walked to the house in the dark (after 5 when Josh left), entered through a door left unlocked by Josh, raped and murdered Susan (he may have taken a trophy after, if so it might unknowingly be in the possession of LE), returned to his car and driven to the airport to fly back to Washington. Josh was probably told how and where to dispose of Susan's body when Steve laid out his plan to him. After hiding Susan's body, Josh could have taken the kids and stayed in the motel room Steve had rented for the rest of the night. Josh would have had a very long day behind him and probably needed sleep. Josh would not be on record as having rented a room and the children would not have seen Steve.
If this theory of events is close to correct it might be possible to ascertain Steve's whereabouts on Dec 6, 2009. Check passenger lists from Seattle to Salt Lake City for the 6th and 7th, checking for round trip ticket purchases. How many males made that particular round trip then? The list might be very short. Find all of them and get a visual by LE. That would be the best chance of discovering the name of a passenger who cannot be unacounted for. Check car rentals from the SLC airport for Dec 6. Show Steve's picture to the motel clerk on duty where the woman believes she saw Josh and the children the morning of the 7th. Lunch was possibly eaten in SLC, but where?
With so much time having passed chances of anybody recognizing Steve or remembering the date if they do are probably slim but every possibility should be checked. If there is recognition, the date(s) to check records are few.
If I am right, Susan's killer is still alive but he is in custody and there is time to check into the what I have posed. I am not a sleuther so my theory may have many holes that can be pointed out but perhaps somebody with experience in sleuthing can patch this into a possible theory.