Yeah, my theory is that the kids were sent to their rooms and told to close the doors and Stacy was taken out and put into that car before it was moved. DrewP is autocratic, if the kids were told to go to their rooms and close the doors, they would have done it.
The one thing that Drewp didn't want to risk was that the kids might go into the br and see Stacy and I think he would have gotten her out of the house right away. There was also the danger that Sharon or Cass might stop over and expect to see Stacy. So her car had to go, so that he could say she left. After that, he started setting up diversionary tactics and at the same time trying to give the impression that it was just another day and he was just another husband whose wife left him.
While he was away from the home that first time, I think he probably was setting up arrangements to move Stacy's remains somewhere. Then after the little ones went to bed that night, DrewP started moving cars, and disposed of her remains.
I think that Cass surprised him, I think he thought he would have one more day before Cass made a report if he couldn't convince her. So that is an advantage, she didn't wait that one more day.
The three days after? I do think that DrewP was doing something related to Stacy. When he was near the home he covered his face so that people on the street wouldn't recognize him. Then all of a sudden he quit hiding and started mugging for the camera.