I'm not sure whether I'm going down the path of some Thomas Pynchonesque-type conspiracy here, but could the police have focused on Spedding so heavily, and thereby allowed the media to focus on him so heavily, to expose his identity far and wide throughout Australia. This could have been done to enable both potential past victims, and past criminal associates, to come forward to detectives with information about things that he may have done in the past. This would at the very least paint Spedding as someone capable of kidnapping William, but at the very most allow him to be arrested for past crimes. If the later case occurred (and indeed in the last few minutes it seems it has), this could perhaps install fear in his associates (read 'ring' members) and coerce one of them to come forward.
I.e. what I'm suggesting is that the public focus on him, and this recent arrest, may have at its heart the purpose of not just legitimately forcing Spedding to confront and suffer consequences for past crimes, but coerce others through fear of being incriminated themselves to come forward with info about William so as to get perhaps some immunity from crimes they have committed in the past? E.g. 'I will tell you what I know about William Tyrrell because I'm afraid Bill Spedding may now reveal crimes I have done in the past considering that he has been arrested. Just please grant me immunity from consequences for doing those things if I tell you what I know about William'?