You make a valid point which I have always said, Bennet and Aiden makes the most sense to the story. As it is blackmail route, Sophia is sort of forced into it and not doing it with her free will as she is doing it for her son's future and safety. Though once it happens, she will begin to enjoy it, and if L&P plays his cards right and does creative writing, he can create many angles that can involve other characters as well with this angle.
Patricia's husband I think has very little interaction with Sophia if I remember correctly, almost non existent. Neil also had potential but after that bathroom episode, it was put on the end. Janitor's story could also have a very good story line as he could emotionally blackmail Sophia with his stories and as he is a huge guy, he can overpower her as well.
There are many angles he can create with prolonging side characters or just adding them for the sake of it.