Going to war in order to unite everyone (under your own leadership of course), has often been justification for war in real human history. That is what Jon is doing here. He has seen the threat and he knows everyone has to be united in common cause against the enemy of all warm-blooded beings.
Of course it would be preferable to unite everyone by using Wilding and Night's Watch testimony, evidence, and inspiring speech, but this seems unlikely in the case of Ramsay Bolton. Heck it
might have been possible with Roose Bolton,
maybe, but definitely not Ramsay.
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As for Arya=Waif, I don't particularly like that theory and I hope it doesn't go that way.
One of the great things about
Fight Club is that once you know the twist, you can watch the movie again and see how carefully everything fits. But in Arya's case, if I think back to earlier scenes, not everything fits right. There are two scenes in particular where it doesn't make much sense to me:
1) The scene
@zmunkz already mentioned. This doesn't make any sense to me with the Arya=Waif theory:
2) Secondly, in the scene where Jaqen gives permission to The Waif to kill Arya, he makes a point of telling The Waif to do it humanely, and that it was a shame because she had so much skill. If you interpret this as Jaqen telling Arya's "
Faceless Man" side to finally kill off Arya's "
Stark" side, it seems awkward to mention the loss of skill and to tell the Waif to kill Arya without suffering.
I don't know how to make sense of either of those two scenes with the Arya=Waif theory.
- it could have worked .... If GRRM had written it so, but and to confirm this I need to binge watch - she sees the waif early on so unless the waif dies when she goes blind and then she splits at the same time I don't see how the split personality works 
