Don't know why so many people think Martin will write the Purple Wedding. Of the "event" episodes, David Benioff & D. B. Weiss have written "Baelor" and will write "The Rains of Castamere". Martan's only event episode was "Blackwater".
At this moment, each season of GOT has started slow and building up to huge events. I also think it will be bad writing to have the trial take so many episodes. You don't sideline your leading actor for that long.