Occam's razor: The simplest explanation is usually the right explanation.

These theories are too complex and will likely fall apart under pressure. The only person who fits RJ's profile is Danny Ruskin. PJ's brother in-law.

He has always been jealous of Patrick's success and angry that he took his sister away from her family. He has access to Patrick's past. He knows Patrick better than anyone else. I know that RJ killed his sister and niece, but he is a sociopath and a con man. He can blend in, and he can charm and manipulate people without remorse.