an amnesty from prosecution.
If he is guilty, he is guilty, so won't come forward.
If he is innocent, he is innocent, but will be worried about being prosecuted.
At least we will get closure.
I don't see it as binary as that.
Guilty of what? Smithman could be an innocent male relative / family friend with unrelated reasons to keep out of the spotlight, whether of an illicit or simply awkward nature.
If, for example, Smithman had been carrying the daughter of his mistress and couldn't be assured of confidientiality in view of the cosy relationship between the police and the tabloids. What then?
Or his working papers may not have been in order, with a family to support. What then?
Or... he may quite innocently not have realised that he may be the person the police are seeking to question.
Or, perhaps he did come forward and his statement got lost somewhere in the mounds of paperwork and has since been eliminated.
I don't find it clear whether the remaining unresolved lead according to recent media reports concerns Smithman or Smellyman, nor whether there is a suspicion that they could be the same person, or possibly an associate.