If M was murdered it probably would have been very shortly before going to sleep, before she had time to change clothes. Did something happen that night that prompted her to sneak out for awhile and could she be the one parents claim to be on the camera?
If she was in jammys it makes no sense whatsoever a killer would take the time to dress her and risk being caught.
Mom said in the radio interview,as she was describing what Steve saw when he went in to talk to Morgan, "...he thought she was in her jammies." But like I said upthread, what he thought he saw is open to interpretation. Her clothes could have looked like jammies to him. IMO, she was never in what one would normally think of as jammies.