@Lash,
I think that if Qyburn was just informing Cersei there would be no point mentioning him. I think what he said was the reason Cersei was so phlegmatic about Jaime's meeting.
Sorry I meant the Mountain, this guy.
Oberyn killed him, not straight away but with poison. The armoured creature at Cersei's side is undead, reanimated by Qyburn which is why I think Qyburn may believe he can deal with the white walkers.
Littlefinger definitely played Arya, and he is trying to set Arya and Sansa against each other. He may have some initial success but I think it will backfire, not least because Sansa knows how his mind works, and Bran also knows what's going on.