Would you arrest him?(obv spoilers)
Put yourself in Andri's shoes for a hot minute; you're living in your in-laws' house, they're helping you out quite a bit (not just financially), you have a great relationship with the father, and then add the ex-wife scenario with the possibility of getting back the marriage you still wear your ring for.
Now, think about what we know; Hrafn and the others had burned down a building for money, but in the process killed Dagny, who wasn't supposed to be there but they were anyway. Pretty bad, but it does get worse. They covered it up so they could claim the insurance, then ruined a young guy's life by sending him to prison and making him think he caused his girlfriend's painful death, used Eirikur's grief to their advantage, then afterwards Hrafn says "Yeah, well they weren't supposed to be there..."
Do you think you would arrest Eirikur given all that you know as Andri?
Eirikur wouldn't have known this but Andri also knew about the prostitution, the covering up of the other dead guy, covering up a rape, etc.
I don't know if I would, I think I might drop that key into the ocean.
"Everything could've been anything else, and it would have just as much meaning."