I doubt that anyone else was involved in the abduction and murder. Christine's family returned home around 4:10PM, she was buying bubblegum at the corner store around 4PM. She vanished in between.
Calvin Hoover must have been at her house, put her coat on the top hook, told her to bring her recorder and hop in his vehicle, then he drove somewhere, assaulted the 9 year old, drove her to a field, violently stabbed her numerous times, cut off her head, and drove 50km (33 miles) back to her house for the search. It has been said that he attended the funeral and participated in searches.
The kidnapping, assault, murder, decapitation and 100 km (65 miles) round trip driving time would have taken a couple of hours.
His wife and children would have noticed that he did not return from work at a normal time and that he was in a different mood when he did return.
Christine was abducted shortly after 4PM. Driving time was an hour highway driving, longer on back-roads. Cutting off someone's head takes time. Rape of a small child takes time. When did he get home that night? I think it's fair to say that he was late for dinner, and probably wanted to clean up when he did get home. Was it not a concern that the daughter of a co-worker went missing when he had a sudden change of schedule? One person, in my opinion.