My current theory about how this all could have happened ...
Abbott was in the area visiting with the mystery man at #35. Who knows why. Mystery man grew pot in his window box? Had kiddie *advertiser censored* videos? Had violent adult movies? Something(s).
Abbott saw William as Abbott was leaving, and took him.
Savage saw this from his back porch, and the whole area knows who Abbott is and what he is capable of.
Abbott makes his way to the school with little William.
Porter picks them up, much later, and drives them away.
Did they drive 300km? I don't think so, hence the searches all around where Abbott lived.
Porter likely said 300km because he knows William is no longer with us, and he didn't want to totally blow his mate in, so he made it seem as if William was passed to someone else far away. Made it seem that Abbott didn't murder William. That way Abbott might go down for abduction, but not for murder. And fools would quit blaming the FP.
Savage is too scared of Abbott to speak, even though they have tried every which way. Reward, immunity, phone recordings, visits on unexpected Fridays .... and more. But he, too, doesn't want the innocent FP blamed so he supported them however he could.
Mystery man at #35 is frightened too, so he moved away.
No-one wants themselves or their families to experience Abbott's wrath when he gets out of prison. And he wasn't in prison when William disappeared.