No, she shouldn't have gotten in the car with him, but she wanted to find Adrian because she felt responsible, and he dangled that possibility in front of her. I don't think of her as dumb but more as being blinded by her guilt and frenzied emotions. Linden didn't want another death on her conscience.
You do crazy things when guilt is messing with your head. Linden's unstable emotionally. I bought it that she'd get in. She got totally immersed in the Seward case back in '09 and always had doubts about them getting the right man. She was involved with Skinner; he said "my wife and kids" and she went off the deep end for reasons including all of the above.
It was dumb to get in the car, but it was earned because of Linden's emotional state. She kept saying, "I should have seen" but she didn't because she liked Skinner. She was so obsessed about not fucking up again with Adrian that she didn't even pick up on Holder in IA being because of Skinner. Her mind was in a frenzy. All the girls are dead and Seward is dead. She saw him drop and suffocate and it majorly fucked her up. I liked it. I really did. Also, some serial rapists (many of whom are married) have been known to give some jewelry from the victims to their wives. They're sick puppies.
I rather like that they didn't find Callie. That's the way it happens too many times. Also, you aren't kidding that there needs to be another season. Holy SHIT. Oh, don't Linden and Holder have a bond now.

You know he'll cover for her and there will be hell to pay emotionally. Devilish and brilliant by the writers, though, to build up that bond this way. Linden's instincts are brilliant, but she's really not fit emotionally to be a detective. She gets too involved and it messes her up. If I were her superior and I knew the facts, she'd be gone.
But it would make brilliant, riveting story with Enos and Kinnamon's chemistry, no? They'd have to pick up right where they left off, that's for sure. Oh, there
must be more.