Here is my take on what may have happened to Allison.
She was killed at home around 9:00-9:30 pm. Another female (OW) came to the house later after GBD asked her to come over urgently to watch over girls (more screams when this person arrives and realised what had happened). He then disposed off ABC at the bridge around 11:30-12:30. I believe he pulled the body off the back of car onto the ground (first thud) and then rolled the body off the bridge onto the mud below (second thud). At around 4:00 am, after conferring with NBC, both went back to move her under the bridge to the location where she was found (more out of sight). Just speculating. My opinion only.