Question about Canadian laws for my screenplay.

I wanna get all the legal stuff as accurate as possible. In my script the police have a #1 suspect for murder. They have no other leads but the one suspect was at the scene. So he's the only guy. He is however, an eyewitness in a high profile case, where the prosecutor hopes to convict the city's biggest gangster.

The prosecutor does not want to loose his witness, by charging him with murder. So he tells the police that he will not prosecute cause there is not enough evidence, and tells them to look for other leads. Will the police call it an unsolved murder then, or would they just go to a different prosecutor?

Are the cops allowed to go behind the original prosecutors back and seek out another one? And is the prosecutor allowed to decide to not press murder charges on a suspect, since he wants to use him as a witness? Thanks.