Question: Was Mrs. Collins' son ever found?
Question: When Mrs. Collins opens the fridge, is there a sandwich in a plastic bag in there?
Answer: There is a sandwich in a bag in there as well as a full jug of milk. It's one of the ways that she knows that her son isn't there.
Question: Warning: Spoiler. At the end of the movie, it's found that three of the victims had managed to escape; only one was actually returned to the parents. It isn't known if the other two were recaptured by the killer or managed to get away (but went into hiding). Why didn't the police just go back and question the killer's accomplice? He'd likely be able to reveal whether any of the boys had been recaptured. (We're never told if the accomplice had died by that point, or whether he was unavailable, or whether nobody bothered asking him.)
Answer: When the accomplice is originally questioned he says that killer told him a few 'may have escaped', implying he never knew if they were caught.
Also, it's implied his accomplice a young boy himself was scared of ending up like the boys himself, sadly I think it was done for survival.
Answer: No. He was murdered at age nine. The movie uses creative license to bring up the suspicion that he could have somehow survived to create a dramatic hope in the end. Moreover, the killer was very unstable and retracted his testimony more than once. There is no solid proof of the boy surviving the killings. The police even found partial evidence of Walter Collins at the burial site. See the Wikipedia article for more information.
Earthling