Actually it is up to the jury to decide what is true and not. I would have no problem believing that someone was afraid and then came forward at a later date.
It's up to the primary detective to determine his primary suspects and to dismiss suspects. if he feels someone is truthful in his statements and he has determined he is cleared, he can choose not to pursue him as a suspect. There have been 1,000s of people cleared of the Zodiac case by Toschi, Armstrong and Narlow..none of their names have ever seen a jury. The decision on whether they remained suspects was the detectives. A person cleared by a detective never needs to see any courtroom. Hell, the detectives own notes never need to be known to anyone.
It is up to the detective to determine among his witness, victims and POIs as to whether they are worth escalate to prime suspect status.