Kjartansson looked a little bit off it against Hibs, not a risk worth taking. We were losing the game in the midfield, so as you say, Nilsen the best shout. Realistically, none of the midfielders (apart from Aouchiche, who does the running required), can really do the job required in a 4-2-4. I'm all for attacking against teams like Falkirk, but we're losing so much in midfield, the extra attacking options just aren't getting brought into play.
Karlsson has worked hard, but it's interesting how a player of his calibre (based on transfer values rather than reality it seems) reads the game so badly. Both Keskinen and Bilalovic went beyond him on occasion, and he just stayed central when they lost the ball. Both times, he had to sprint to get back in to cover, when he should have been in position ready to hold up the fullback ages before.