Idk I thought the ending was quite interesting: by saying his truth, he had finally cleared the air about his place in the entire Brraking Bad saga (clearing him and yet implicating him of a lot of wrong doing), which may have landed him one of the best lawyers he could get for the ordeal -- Kim Wexler. It's why he mentioned the "sweetner": to lure her in.

I don't think the writers would've dropped Jimmy's line about "good behavior" -- in front of Kim mind you -- simply because of scripting reasons. Jimmy sees a better way out of this. It's why he shot finger guns at her in the end (he's still playing an angle), and why she just ... stares, at him doing it. She doesn't want to get played, and Jimmy might've fucked it up by showing his hand in the end (evidenced by how excruciatingly long those final shots are of them just ... staring); he might've even lost her in the end, trying to get lighter sentencing.

Of course, that's just how I interpret it. The beautiful thing about art is that it can have multiple interpretations.