Who is the serial killer in Memories of Murders?

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.

From Wikipedia:

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.

If you pause at 48:23, you can see the killer. See the pic attached.

At around 1:24:00 the song Sad Letter is played on the radio, and this is the last time a murder is committed in conjunction with the song and rain. While this playing of the song led them to the letter and then onto the prime suspect, and was the reason the song no longer played a part in the murders, it should be noted that the radio's stations program producer quit suddenly on this last occasion.

The producer had been alerted to the police interest because they had already visited the station when the song's link to the murders was noted. So it was the radio stations program producer who committed the murders, but found that without the song it wasn't of interest any more and stopped after a few more. Although, he could have quit when he realised that he'd allowed the song to be played and that the result of this would be another murder.

I'm still going to say it was the producer, but that's the movie mind you, the real case has had officers worked for 2 million man-days on it and there are 21,280 suspects.