I don't know. It could also be that Arya just kills Cersei before Danny even comes.
I suspect this may happen. In fact, it may be how they can take King’s Landing without Cersei blowing it up. Arya might not even be involved with the war. She just wants to get her revenge. I forget what her list is like, though it’s getting smaller and smaller. I know Cersei was definitely on the list. I’m not sure if Jamie was on it or not. The Hound was, but she might not be interested in killing him anymore since she already did (sorta). So she may just make her way to King’s Landing with the intent to kill Cersei and possibly save the day for Jon and/or Dany.
It seems like they’re focusing on Arya’s revenge enough that it seems unlikely she won’t be the one to kill Cersei. But then again, how poetic would it be if Jamie once more commits regicide—and against his own sister/lover too. I mean, that story’s way better than Arya’s revenge tale.
Maybe it’s both. Jamie demands she steps away from the brink of madness, she refuses, he has to fight the Mountain, and Arya sneaks in during the fight and strikes her down just as Jamie kills (or has a pyrrhic victory) the Mountain. That seems far too simple, so that won’t be the way it ends.