This looks like most Japanese mystery/murder series. In which some extremely hard to believe things happen. Usually, its a murder cult able to take control of the police and stop all investigations into them.
Here? Its a mysterious mangaka that has brainwashed multiple people into helping him commit murders to the point where they would gladly sacrifice themselves to keep the conspiracy a secret. Even the MC's girlfriend is part of the conspiracy and cut off all ties to her family and friends just to keep the conspiracy a secret, while instantly breaking up with the MC because the murder conspiracy is the most important thing in her life.
This is not believable by any stretch of the imagination. It is a blatant plot device because the author couldnt come up with any better ideas.
