It's an interesting theory, the guy sounds like a total POS, but the investigative steps weren't well-thought and there was a lot of self-aggrandizing.
One aspect featured in episode 3 if I remember correctly was when the author visited the roller skating rink where two girls were last seen to interview the woman who had last seen them. The author shows the woman one photo of the suspect (Jim) and the woman agrees, 'yes that's him.' 1) There's a reason the police use a lineup of several photos, not just one of the suspect. It's to ensure the witness can accurately identify the individual. 2) The woman says 'yes that's him' and goes on to say something like he was skinny or slender. In other instances, the author notes that Jim was a football player with a stocky build. Instead of pushing back and saying maybe it's not him, the author just agrees with the woman and goes along with it. I got the sense that the author wanted it to be true rather than performing due diligence and evaluating what the woman said.
The author's mother, Jim's stepdaughter, said that Jim's guns were turned over to the sheriff when he died and they told the sheriff about their suspicions. Why didn't the author follow up on that lead?
Any man who violates women like he did is a POS. His behaviors suggest he is definitely capable of murder, but all of the "evidence" provided was speculative.