To play the devil's advocate, and why I was originally skeptical of this, faking her death in such a manner would gain Arya very little considering the risk involved. Since the waif is a far superior fighter, trying to manipulate a surprise attack like that would probably get her killed, even if she could be sure of exactly where and how the waif would stab her. And it's not as if the FM are careless; if they really want her dead they'll be highly suspicious if no one actually witnesses her dying and the waif can't account for her body or face. But more importantly is the fact that if she gets out of Braavos without faking her death, she obviously would need to lay low for a very long time after getting to Westeros (as she'll obviously have to do regardless of the Faceless Men). On the other hand, if she fakes her death, she'd be doing the exact same thing anyways, since once word got out that Arya is alive, the hunt would be on again. Having the opportunity to escape, it would make little sense to try to one-up the FM in one last machination, especially considering her past failure to either deceive or hide any aspect of herself from them.
