If the girls were alive when SW arrived home, three hours late, and the window for the murders had narrowed, why would CW risk the murders that night? Why rush and make mistakes? Barely three hours was not enough time to do anything more than what he did do.
CW thought he was smarter than everyone, so I believe he had a plan. With the delay, and less time to act, why not wait a day, or two, to do things according to plan and avoid mistakes and risks? The only logical reason is, he had already murdered the girls and had no choice, but to continue.
As to screams; I'm sure he knew he could easily overpower SW before she could scream, or do anything. Murdering Shanann had to have happened soon after she walked in to the house. Remember, she still had her make-up on.
MOO