I'm about to run this game soon and this is how I'm going to handle research:
1. I'll determine if any of the time spent with the Professor would bring about one of the research topics. (For instance, if a fighter was his bodyguard while the Professor was traveling, he could have conceivably heard ghost stories about the Prison from the Professor.)
2. The "once-per-character" knowledge check would only be able to recall knowledge DC 10-15. If they have a back story that might work with it, DC 20 would open up for that character, otherwise it would be research-only.
3. All DC 25 knowledge checks require research. Unless the character is a scholar or heavily experienced in the matter, they probably aren't going to know that high of a DC offhand.
Furthermore, you aren't going to find any specific information on the Five Prisoners anywhere except the Ravengro records and the in the records of the towns these Prisoners are from, and it's already clear that "gather information" (ie, word-of-mouth) only gets DC 15 or lower in the very town that experienced it. So being a stranger to Ravengro and most likely NOT from one of the areas where the Five Prisoners were active, it is highly unlikely they would get that information.
Does this make sense? Research is my job as a journalist so I think all of these would be realistic and be able to stagger out the information gathering.