Jack, Susie's dad, is the frustrated hero of the piece. He's the loving father who knows the truth of his daughter's murder, but can't prove it. His struggle is perhaps most poignant. He sticks it out every inch of the way, facing the truths of his world, and trying to keep his loved ones and himself from going mad over what's happened to Susie. He even manages, to some degree, to hold down his accounting job. Like the rest of the family (excluding perhaps Abigail) he believes in Susie's ghostly projections and listens to what she has to say. He, like Ruth, is comfortable with the idea of the dead and living coexisting.
Like Susie, he's a distinct foil for Mr. Harvey. While Jack's journey is a process of rebuilding and recovery (to whatever extent possible), Mr. Harvey's is one of disintegration and decay. Also, think of the bizarre moment when Jack is overcome with vengeance and almost attacks Clarissa in the cornfield, thinking she's Harvey. He almost inadvertently becomes a version of Harvey, a man who hurts young girls. Clarissa and Brian sure see it that way and use it to gain popularity among schoolyard gossips.
So, yes, Jack Salmon is something of a whipping boy. He gets taunted by his daughter's killer, baseball batted by Brian Nelson, cheated on, and heart attacked, in that order. He's also not perfect. In a disturbing moment, he all but begs Lindsey to break into Mr. Harvey's house. And because he can't bring himself to overtly involve her in such a dangerous thing, she goes it on her own. He doesn't pretend to be aghast when she brings back the proof, but thanks her sincerely. Man, if Harvey had gotten her, Jack would have probably died of guilt.
Interestingly, Jack never learns of Abigail's affair with Len Fenerman. What's more important is that Jack learns, in Abigail's absence, a lot more about who she is. This strengthens his love for her and gives him the knowledge he needs to develop a deeper relationship with her when she returns.