I have a theory. Kenneth's murder still bothers me. I can't see a vehicle with all 4 killers going to his place and all probably still with a lot of visible blood on them. Kenneth's dog, Brownie, might know the killers but how many of them can he know and like really well... and wouldn't 4 people showing up cause him to be nervous and bark or growl even if he did know them? I know many think Jake killed Hanna but I'm wondering if he could do that. Anyway, what if he dropped the 3 others off at Dana's house and went to Kenneth's on his own. Maybe he had made good friends with Brownie over time... maybe even used a "clicker" device to train Brownie to expect a treat when he heard that sound. Or... maybe he actually called Kenneth with some ruse... car not working right, whatever. He got himself into Kenneth's house one way or another and we know how that ended. Meanwhile, the other 3 are killing Dana, Chris Jr and Hanna... and even take the time to clean themselves up... remove protective clothing and bag it... wash off any visible blood, if stopped by LE, they won't be bloody on the drive home. Jake drives back from Kenneth's place and removes his protective clothing, bags it... checks for and cleans off any visible blood. They pull up any plastic placed down in the car and bag it, too. All clothing and plastic goes into the spare tire area of the vehicle. They head to the highway and they're home very quickly.. already clean and with bags of clothing to bury, burn or otherwise destroy.
I can't help but think heading away from their own house at the end of a chaotic night just to kill one more person as a unit of 4 doesn't make good sense to me. It would take too much time when it's time to have this all over and done with. It's taking a terrible chance just when most people would want to be escaping after having killed 7 people. I think, somehow, Kenneth's murder was done "inside" at the same time as the other murders by one of them or, by someone else entirely who had no other "job" to do but kill Kenneth.
(... or Jake doesn't go into Dana's house at all upon his return but just picks the other 3 up... bags his clothes after leaving Kenneth's house. I just don't think he could do it.)