If you are saying shouldn't they have claimed "it was an accident" and negotiated a deal a long time ago, to get the best deal, then yes I certainly agree. But that's only if it was really an accident and he told the whole truth, because he would have needed not only a willingness to bargain but also something solid to back up his story.
Since about a week after she went missing, imo there has been no point at which the state would have simply believed his word for it that she died with him but that he didn't mean to. They had too much that said otherwise. And they can't legally bargain an abduction-murder down to an accident if their evidence says he's lying and it was much more than that.
But that was then, and that ship has already sailed.
Right now, I doubt he can gain much in relation to the AK by fessing up and telling what happened to CM, because he's already toast on this crime. But I think he can keep from having a murder charge and the possibility of a DP hanging over his head for the rest of his life, and that might matter to him quite a bit. And until the jury slaps a guilty verdict on him with a sentence of life to 99 years, the tiny bit of uncertainty there does give the state a small bit of motivation. But afterward, assuming he gets locked up for a lifetime or two, not so much.