Eh, there's lots of reasons Zenos could turn on Fandaniel - most notably, if any of Fandaniel's machinations threaten his violent reunion with the WoL. If such an occasion arises, however, I see him as being just as likely to stab him in the front than in the back. You know, whatever side's convenenient at the time.
I agree, though, that Fandaniel could come up with an assortment of reasons to stab Zenos in the back (which would, of course, be another prime motivation for Zenos to stab him instead), but I doubt he would be successful. Zenos is the
Villian Sue to the WoL's
Mary Sue - the only one in the game whose unexplained invincibiilty and powers of plot contrivance could possibly rival our own! Wacky, capering Ascians may come and go (RIP, Emet), but Zenos will never die (for good) until he has his Ultimate Showdown of Ultimate Destiny with the WoL.
The foreshadowing of this is so strong, that even if they DID pull a curveball and have Zenos bite it before the final act, I can't help but feel that it would seem... weird.