No, it isn't known who the killer is. The film is based on a real-life event, the Hwaseong serial murders, which is an unsolved case in Korea. The film too can therefore only end on a similarly unresolved note.
In the end, the crimes remain unsolved. Visiting the crime scene years later in 2003, Park Doo-man, now a businessman, learns from a little girl that the scene had recently been visited by another, unknown man with a 'plain' face. The little girl had asked him why that man was looking at a drain (from the scene of the second murder, which is shown at the start of the film), and he told her that he was reminiscing about something he did there a long time ago.
The final scene where Detective Park stares at the camera is open to interpretation. But, one of the interpretations is that he is convinced that this man with the plain face must have been the killer and that he could be just about any Korean. Park then looks through the camera at the audience trying to locate the murderer as the Hwaseong killer is surely watching the film himself.
Also, the factory worker who was the prime suspect could well have been the killer in the film as the DNA test results from the US do not exonerate him. They simply state that the evidence is inconclusive.
I have no idea about the OP's question on the credits for the half-second scene. But it is unlikely that such a credit has been listed.