Question: I understand the concept of spelling mistakes and an editor accidentally missing them, especially for an unknown actor's name. But how did "Callahan" end up getting misspelled when it was spelled correctly in the first film? Has any reason been given? Or if there someone with general knowledge of how end credits are produced, is there no editor?
Question: I'm not sure if he already suspected them at the time, but why exactly did Harry want to take Davis and Sweet to arrest the gangster?
Answer: Because Davis and Sweet demonstrated keen interest, skill and knowledge of firearms (particularly of Harry's .44 Magnum) when they first met. They were flattering Harry, probably in anticipation of inviting him to join their death squad. Harry was impressed and asked for them specifically, even though they were rookies, but he didn't suspect them of being the vigilante killers at that point.
Yes, he did suspect them. In fact, he suspected Davis from the moment he found out that Davis was the one who reported Charlie McCoy's death. He confirmed his suspicion about Davis after he compared similarities between bullet that killed one of the victims and the one he purposely misfired from Davis' revolver during shooting competition and retrieved it afterwards. He explained it all to Early Smith right after raid on Palancio (and thus unwittingly sealed his partner's fate).
Chosen answer: The error didn't necessarily start with the makers of the credits. It could have crept into the process at any point. Whoever typed up the names to give them to the technicians who made the credits could have misspelled the name.