I'd argue playing Karlsson would be more difficult than any other D on this team. That's not a knock on Karlsson, just his style is unpredictable. Honestly I dont know what he will do, even after 300+ games of watching. Add on the fact that he makes high risks plays. You are right though, he needs someone who knows their role, I don't think Cowen is that guy yet.
And that's maybe why the guy with the simplest game, Boro, is the best fit.
Boro plays it safe. Boro knows his role. Boro doesn't rush the puck. Boro passes the puck out of the zone competently. Boro stands up for his teammates. Boro has limited mobility - so have him cover less ice by always being the safety.
Methot is still a much better fit - his mobility is better, so he's a better option on quick transitions against and on odd man rushes against. Methot moves the puck better, especially in the offensive zone, which adds a dynamic Boro can't.
But it may be a move similar to putting Stone with Turris and Mac, and Ryan with Zib. It creates more depth. And Cowen can play his game, rather than trying to reinvent himself and always read off EK - which is difficult to do for him. It's enough for Cowen, it seems sometimes, to remember where he's supposed to be, add onto that the read and react to your partner - and you get a guy who blows the zone like he did in the first game and are left wondering, "where the **** was he going?"
If the staff does quit on the idea of Cowen with EK, which is still a very new pairing and deserves more time IMO before we say it doesn't work (hasn't yet, not betting on it, but still early), I'd be interested to see Boro with EK. Heck, I'd even like to see PW with EK. Patty's game is actually not that unlike Kuba's. Decent offensive skills, a little slow of foot, good hockey mind... PW isn't Kuba - but he might be the next closest thing we have that is currently in the NHL.