Just completed this game, and while I enjoyed it I'm full of questions regarding the plot. Some of this I missed or forgot due to having paused a few times for longer periods while playing, and some I believe just wasn't explained in the game. So if someone could help out and give a few answer, that would be great!
I just finished the game as well, there are probably people more suitable than me in these forums to answer these questions but here's what I think I know:
1. When they first arrested Keith they thought he was a victim and not the Cain Killer. McCoy probably escorted him outside to wait for help, and then returned to Erica.
4. The game mentions that in the initial will, the belonging were divided by three and left to all siblings, but then Skorobeus forged the new will.
6. They are investigating Max for financial fraud of some short. McAddams mentions to Cordelia that Max inherrited a large sum from someone named Jeff Therrien who doesn't appear to be a real person.
7. Keith said he was always a psycho but he was able to suppress it. However the night he learned Max and Cordelia were not his real siblings, he felt isolated and angry. He was also always jealous of Cordelia's and Max's relationship which is why he began targeting brothers and sisters.
8. Cordelia was in jail when Keith commited the murders. She was able to see the future so she most likely knew about them. Once out of jail she predicted Scott's murder and told McCoy. She also tried to stop it but arrived too late.
9. Cordelia captured Keith after the Reed murder in their parents' lakehouse. You actually play this segment in the beginning of chapter 4.
10. She wanted to kill the agents who didn't stop her brothers murder but couldn't kill Keith and tried to push Erica to do it.
11. McCoy mentions at some point new evidence appeared that proved Cordelia was innocent. From what I understand they covered it up to hide their failure to properly sovle the case.
12. ^^
I am not quite sure about the whole financial plot between Max Scorobeus and Keith. Someone else will have to answer that.