It is not stated but to me, they really look alike :
glasses + scar at the same eye... well I didn't even question it before you said you didn't see it that way, so to be fair, maybe that was a hasty conclusion
And on my second point (sorry my English is not great, I'm french :) ),
I am saying I don't think in chapter 251 meant he literally had to die to know the truth.
I was refering to chapter 249 with Allen saying goodbye to Kanda after they arrive in front of the library saying "I'm leaving !" (I have the french translation), with a kind of smile that seemed to say "farewell" + thought bubbles saying "he was somewhat remodeled by the memory change + not even sure that the "true allen" even exists + "will I even be able to be "me"
+ Mother praying for Allen saying "This kid will never come back. Farewell Allen Walker"
So to me it's more like now that he will know the truth, the "Allen Walker" as he is now will never be the same, because he was shaped by his past trauma and experiences, which could have been modified. His whole value system and what he believes in may change etc.
So what I got is that the price to pay to know the truth is just that, not really "dying" per se