Personally I think Edlund took the idea of crazy!Cas a little deeper than Gamble did. Take the board games for example, when Cas tried to get Dean to play Sorry, he was conveying a message to Dean. He was apologising when he held up the board game (obviously), and when he told Dean "it's your turn to move" he's telling Dean that he has apologised enough, it's Dean's turn. When he said "You have to go back to the start", he's telling Dean that they can start from scratch and leave the old grudges behind. At least, that's what I interpreted. When Gamble threw in Twister, that looked to me to be a regular board game with no significance, so it seemed to me as if she couldn't grasp Edlund's meanings behind the motives. As well as that, Misha confirmed that crazy!Cas is the way he is because he's trying to suppress the guilt he was feeling, but turning away from conflict and abruptly changing the topic every time any of his past actions were brought up. It seemed that he was back in his old self in Purgatory though.
Personally I liked the leviathans because for once, we have a villain who's smart and knows what he's doing. I was a bit disappointed that d**k was killed so soon, but who knows, they might see him in purgatory.
As for "cut off the head and the body will flounder", the rest of the levis are still alive, as confirmed by Crowley in the finale. However, he did say that the levis have only had one leader since the beginning of time, and without d**k they would be lost and be just regular monsters - or in a sense, "flounder".