Question: Does the main character(s) in a murder mystery, more specifically the narrator, need to be a detective? Can it be someone who is in a completely different occupation but has as good as a chance as an actual detective of finding out who the murderer is? And would do it be too confusing to add a detective in the book who has his own insight on the case, but who isn’t working with the narrator?Answer:
The non-professional or amateur detective is an established figure in murder mysteries. Once upon a time, most female sleuths had to be amateurs, since there were so few women in the police. Some well known examples are Agatha Christie's Miss Marple, Kerry Greenwood's Miss Fisher, and of course Nancy Drew. Some of them are true amateurs, others are private detectives.
And there are plenty of male amateur detectives too such as Father Brown, Brother Cadfael, Lord Peter Whimsey, and the Hardy Boys.
Sometimes amateur detectives compete with the police detective to solve the crime. In some of the Miss Marple adaptations, the police start out annoyed with Marple's "interference" in their cases, until they realize she is brilliant and has specialized knowledge (an insight into psychology derived from a lifetime studying people in her village).
In fact, specialized knowledge is often the key to a good amateur detective. The police have their formal methodology and access to forensics, but they cannot know everything, and sometimes an amateur detective can gain specialized knowledge through their jobs or hobbies. Or they may have privileged access to circles where the police are less trusted. People may tell an amateur things they wouldn't tell the police. Amateur detectives are popular because some readers love seeing a layman out-think the professional detective.
Worth noting... amateur detectives are common in "cozy" mysteries -- stories that are less disturbing, less violent, but good fun nonetheless.
I will say that it is generally better for the main character to solve the mystery -- whether they are amateur or professional. This follows the general principle in fiction that the main character's choices must determine the outcome of the story in order to make it meaningful. So if your amateur detective is the main character, then he/she should be the one to solve the case, and the police detective should be either a rival or ally.