I think what CW's parents have said in these interviews paints a picture that is still consistent with CW being the only murderer that night.
CW said he was the calm one (like Bella) and Cece was 'gung ho' (like Shanann). CW's parents say he's always been calm.
Strangulation as a murder method is associated with a relatively calm, determined, quiet and deep loathing of the victim. If Shanann had got upset about CW telling her he wanted a divorce, is that how she'd be feeling about the girls? But that's CW's normal demeanor. So these murders were carried out in a way that is consistent with CW, from the mode of death to the cover up and the lying.
CW's mom has an image of CW of him as a little boy and a teen playing sports, someone who no one ever said a bad word about. What if his choice to plead now, before trial, before even a probable cause hearing where the DA would present the tip-of-the-iceberg version of evidence against CW, is because CW doesn't really want his parents to hear all that evidence, to hear that he's no longer mommy's perfect boy, that he's become a stinking, cheating, lying, conniving, cold-hearted murderer. Maybe CW doesn't want to go to court and have all the evidence against him presented and turn around and see the look of shame and disgust for him on his parents faces.