i don't think anyone really thinks that Arya will kill everyone from the boltons to Cersei, with the Hound, Robert Stron and Walder Frey in between, it's all speculative and probably wishful thinking in some ways. I do think that I wouldn't want to be on Arya's list though, and that she's closer to completein her training than you're suggesting. Keep in mind that she's been at the temple for quite a while now, and has earned her Black and White robe. I don't think that's a small thing, and to me it suggests that she's closer to finishing than some might think.
I think that of the people on her list Cersei and Payne have the most to worry about.
BTW, being prepubescent, Arya could eaily become a boy, at least if she was clothed. SHe's already assumed the idenity of Arry and gotten away with it. As a FM she could pull it off even better. Perhaps take the shape of Tyrion somehow, and kill Cersei as him to fullfill Maggie the Frog's prophecy.
It's probably not possible, but it's fun just to speculate.