As for Jaime and Brienne, I'm sad it turned out this way as it completely nullifies Jaime's redemption arc. However, it isn't necessarilly bad. It is actually very realtable that he would fall back to his life long addiction. Sad, but relatable and it isn't nonsensical.
I think you may, like Brienne, may have misinterrupted what Jaimie was saying. He is not rejoining Cersei. He is mustering the courage to kill her. His list of misdeeds and "I'm a monster" is to gird himself to murder his sister.
That would make me happy if it turns out to be this. Make sense too.
This being said, that problably explains why the show runners didn't want to use Aria to let her finish her list. It is by FAR the best option available, but it would prevent Jaime killing Cercei or something like that. Again, it smells like emotions and spectacle will triumph over brain... a trend I hate more and more.