My only problem with the doary theory is that Volsemort is, according to the narratives, already back and in the castle. Meaning that the diary won't bring him back . . . and technically shouldn't exist anymore anyways, but hey
And let's stop playing pass the Horcrux, lol. It's likely that the diary is meant to stay in play for a while to cause some mischief, and we'll get the ability to destroy it later. Technically a Death Eater could use fiendfyre on it, but . . . I don't see why they would.
Anyways, Banish No One
Pretty sure I haven't voted yet.