I think this comes down to the game being different from what you're looking for. Henry simply isn't a hero, he's a selfish person who has a good heart, he's flawed, like all of the games characters. You have choices, but they're choices that Henry would make, not yourself.
I was reading an article earlier (on Polygon I think) that describes it perfectly, "You are Henry but Henry is not you," which is pretty jarring when it comes to games. It was a concept I came to wrap my head around while playing it.
As for your second Spoiler, that also comes from you trying to play Henry as yourself, which is not what the game is about.The "crime scenes" turned out to not really be crime scenes at all, except for maybe Brian, though it's hard to know if Ned did it or if it truly was an accident, another thing from which to draw your own conclusions.
I really think if you can put yourself in a different mindset, you'll enjoy the way the game ended more, or maybe not, I can't know for sure.
I get that Henry is his own character and i have only a minor impact on how he turns out in the end, but there are essentially two henries, one who is trying to solve what's happening and one that doesn't really care. There are many points in the story where you can choose dialog options that reinforce that side of him trying to solve what's happening and even D encourages you to take pictures at the campsite of those teenagers. Even if the mystery was a fairly mundane thing they didn't wrap it well enough in the end, which i feel could have been solved with few more minutes of dialogue or a few choose your own text fields.
And i agree that if a person who is playing the game isn't looking to solve what's happening and cares mainly about Henry and D you will get a lot more out of the game in the end, but for a player like me who got intrigued by the mystery, the ending was just lackluster, they wrapped relationship of Henry and D but everything else was kinda left hanging in the air.