The Killing key card doesn't work
Linden and Holder seem to have made a bad assumption, which means the writers made a bad assumption. They find a magnetic key card with blood on it (Rosie's?) after tearing up the floor of the room in the casino where Linden saw it. I assume it's not a badge or ID card exactly, because they never clearly showed if there was someone's photo or name printed on it. They take the key card to Seattle city hall and try it at various doors. The latest clues seemed to point to the Mayor organizing a conspiracy, but the key card won't open the door to the Mayor's office. The card does open the door to Richmond's campaign office. We then cut to Jamie and Gwen, Richmond's two highest campaign advisers, watching their boss at a rally. If they reveal the killer in the final episode, I assume it won't be either of them, because they could hardly have made this red herring trail more clear if they had neon arrows pointing at them.
And if you didn't expect a lot of red herrings in this show, then you weren't paying attention. In the first five minutes of the first episode of this show, we see two red herrings, not even police-clue red herrings, but tactics to mislead viewers. We see a woman with red hair jogging through the woods. Then we hit pause on Silence of the Lambs and switch over the to channel where the first episode of The Killing is starting, a woman with red hair jogging through the woods. Intercut with her scenes are shots of Rosie running away from someone in the dark, presumably the night before. The jogging protagonist stops and sees something by the shore. Was it just my dying tv going dark, or did she see some kind of dead animal on the beach? Or was I just thinking she'd find Laura Palmer wrapped in plastic? To me, they built it up for her to discover the corpse, and then they faked out. She gets a call to come to a crime scene.
She gets to the crime scene and talks with cops who make the usual crime-scene chitchat we're used to from endless police procedurals. Then she gets into the building and in place of a corpse, they have a sex doll. Surprise, happy retirement party for Linden. They lead you down a path, then fake you out. After seeing that, why would you think the show is going to have straightforward resolutions to any of the mysteries?
The problem in this latest episode is that ID badges or key cards are not permanently set. If you get fired, they can reprogram the sensors in the building so your ID key card doesn't work anymore on any of the doors in the building. All they proved with their test is that at the time our heroes checked it, the card is set to open Richmond's office and not the Mayor's. They haven't proved that it always worked on Richmond's door or never worked on the Mayor's door. It's hard to judge how much time passed between the casino chief finding out they had the card, and the time that Linden and Holder test the doors with the key card. But the scenes go from daylight to dark, and it shows several scenes of different characters in different places. It comes after we see the casino chief calls to warn someone (which appears to be the Mayor), after the Mayor has time to summon the shady police Lt, who orders the arrest of Linden & Holder. Plenty of time for the Mayor or somebody to reprogram it so the key won't work, or to make it suddenly work on the wrong office doors.
Also note that this is 24 days after Rosie's murder, according to the title card that shows near the start of the episode. Even if the owner of the ID key card didn't know they lost it at the casino or that it was incriminating, even if they think they lost it at the mall, it's standard operating procedure to report a key card lost. At that point, security will reprogram the locks so that the lost card will not open any doors, and they'll issue a new key card to the person. That would happen passively, any time a person reports a lost key card.
I think this is a mistake, not a conscious decision by the writers. But the show is so full of red herrings and steering away from suspects that have been built up, I wouldn't be surprised if they decide to switch back somehow and explain that the key card system was reprogrammed. Whoever seems least likely to have committed the murder, but who has had a reasonable amount of screen time, is whodunnit. Maybe Jamie or Gwen. I would almost guess Richmond, following the Law & Order Biggest Guest Star rule. Any time a really big movie star guests on Law & Order, they aren't doing it to play a really juicy role as witness or murder victim or minor character. Almost without fail, the biggest guest star turns out to be the villain, the focus of the episode, the second most screen time next to the regular cast, a role that lets them chew the scenery.
In The Killing, the biggest guest star is The Rocketeer, Billy Campbell. They've steered us away from Richmond, given him a decent alibi, and made him a more or less sympathetic character. Maybe he managed to jump off a bridge in one town, get pulled out of the water and drive back in time to murder the girl? Maybe he paid that fisherman to claim he jumped off the bridge. It could be even more satisfying because after you get over your surprise that he did it, you'll remember all those hints they dropped in the first season, and you'll say, "I knew he was shady all along!"