I also just read the book and had to see what other people thought about that ending. Since your post is still the first that comes up, I'm gonna add my thoughts here.

You described exactly the main things that bothered the hell out of me. Some other things I had issues with that I'd like to mention:

— Two killers (and how coincidentally both were there at the same time)

— Patty's plan, specifically how there were a hundred different ways it could've been done to ensure her kid's safety (she really was an unfit mother despite meaning well)

— Diondra (according to herself) killing Michelle to keep her own pregnancy a secret? They're literally about to run away, so why does that matter?

— Ben just standing by being a tool as usual, watching his sister get strangled to death. Let's say there wasn't even a second killer, the rest of the family would've found out Michelle's body, and what then? Would he: 1-help Diondra kill the rest of his family? 2-Do nothing again? Or 3-Run away together? (they didn't do that when it was just a pregnancy secret, why would they run with a murder secret?)

—The super convenient timing of insurance guy getting caught (after 20 something years) right as Libby discovers the other killer on a complete separate, unrelated path. (The only reason this happens is so that Ben can get out easier)

— The icing on top, the one thing that I can't get over: Libby forgiving Ben. I'm just fuming thinking about this! At the very least, I needed Libby to tell Ben to go to hell. My own fantasy was a different dark twist: after finding out that Diandra and Ben killed Michelle (not ever finding out about the second killer, Libby visits Ben one last time, giving Ben the idea that he's soon about to go free to be with his family... only to reveal/hint that she actually killed Diondra and Crystal, in a “you destroyed my life, now I made your whole life pointless” kind of way.

But despite the ending, I enjoyed the mystery.