life is a single skip for joy

I've suspected it was Terry for a while now. Shortly after her affair with Ames came out but I became more and more convinced when it became apparent that she's the link between Rosie/the Larsens and the political side of things. I imagine it went pretty much as you said: Terry believed Ames 'killed' Rosie and helped him cover it up. I'd like to believe she saw Ames chasing after Rosie and followed them with every intention of saving her niece, only to then believe Ames killed her and decide to help him cover things up because she's already lost one person she loves and she doesn't want to lose Ames, as well. I also think she's one of the few potential suspects who'd actually be a surprise if revealed as the killer. None of the people in that elevator would surprise me, nor would Darren or the Mayor or most of the other suspects. And also, Terry being the killer could maybe be a way to keep the Larsens somehow involved next year? Like maybe they do a split between the new case and Terry's trial and how the Larsens deal with the knowledge that someone they loved killed Rosie.

IDK...This is all just to say I agree with you that it was an Ames-Terry team-up.