I listened to the podcast about how the undead Mountain could join the Kingsguard when it had 7 members(Jamie in Dorne, Meryn in Braavos and the 5 seen with Tommen at the Great Sept). Lets review really quick. In the books the members at the beginning of the story were Barristan, Jamie, meryn, Mandon Moore, Preston Greenfield, Boros Blount and Arys Oakheart). Over the show all are mentioned except Arys. Lets assume as happened in the books that Arys is the one that escorts Myrcella to Dorne.
Season 1 - Barristan is removed. The Hound takes his place(which is a big irony due to his contempt for the hypocrisy of knighthood).
Season 2 - Mandon is killed by Pod while trying to assassinate Tyrion. The Hound deserts at the Blackwater. No replacements named. No kingsguard were killed during the riots unlike the books(Ser Preston)
Season 3 - Nothing of note except the infamous Bronn vs. the Kingsguard(Meryn and one other) moment in the first episode. No known losses.
Season 4 - Jamie goes over security with Meryn and Joffrey. He posts Preston, then has Boros guard the entertainment(actually kind of funny if you think about it compared to the books)
Season 5 - Obviously Arys returned from Dorne. If he was alive, Jamie would team up with him. But he clearly wasn't with Myrcella. If he were dead, I think it would've been mentioned as well as a good reason to recall her. So we have Jamie there, Meryn in Braavos and the 5 at the Sept with Tommen. Here is how it works...
Episode to episode, what was happening in Braavos was weeks or months before in Kings Landing. It was fairly clear that Meryn and Mace were only at Braavos a few days at most. Also, we are not sure how long Cersei was in custody before her Walk of Shame. So I suggest, Meryn gets killed. Mace heads back to KL. When he gets back he breaks the news to the council. Then finds out about his children(which is why he wasn't waiting for Cersei with the rest of the council). Qyburn submits Robert Strong as candidate in the name of the queen mother.
So did I leave any holes in this theory?