Bonehead mistakes are part of what Karlsson is, and that's fine (some people have unrealistic expectations that he'll play flawless D and be the most prolific offensive D in the league). The problem is the good is negated when he's playing with scrubs.
Boro is a capable 3rd pair player. No more, no less. He's not an elite 3rd pair D, nor should he be waived and sent to the AHL. Dupuis is definitively a higher caliber player, I really don't see how you can think otherwise.
Methot is an average top 4 D, remember, he was pushed out of Columbus because of a numbers game. His first year hear he was forced into playing a shutdown role with Gryba and looked pretty good. Last year was disappointing. He's not at a level of a guy like Josi, or Hjalmarsson, he's more like a Boychuk. Don't get me wrong, I like him, but I think you may be overrating him, or perhaps underrating Kunitz. He's been a 20 goal scorer, 50 pts player pretty much since entering the league.
We've gotten a shortened season of Methot playing at a level where an argument could be made that he's a notch above Kunitz, and even then it's debatable imo.