MN is there to advise and the A's didn't take his advice. So, MN said "bye-bye". IF the client won't take the lawyer's advice, there is nothing more the lawyer can do for the client, except to release himself from representation.
I too think that the long, winding statement is over the top. He should have just stated that they had all decided to end his representation of them.
And no, he can not divulge anything said to him - ever, unless he wants to be disbarred.
It is simply a matter of a lawyer with good intentions trying to assist someone and then they turn into the clients from h*ll. They are obstinate, go against his advice and expect him to stand by their side while taking a lot of from them.
He no longer believes in his clients' cause or has come to intensely dislike his clients such that he can not summon himself to champion for them. He believes that Caylee is dead, understands that they will never give up hope and wishes them well.
I am sure that CA told him that if he is not on the Caylee Alive Team, he was out. She got a better offer from another lawyer or thinks she can handle everything herself.