Sorry if I was maybe a bit too harsh in my comment. Seeing as you guys had already received plenty of (well deserved) praise for your work, I mostly wanted to focus on my personal gripe with it. But I just want to underline that it is still a very good story, and I did very much enjoy the ride, so you have reason to feel proud of what you've accomplished!
But to address your question, I'll try to explain better what I meant. Basically, we start with an already dead Mori, seemingly at peace with his situation: he stares at his body as a ghost, free of the old shackles of his body and curious to see what this new life after death entails.
This is why I think this "second death" feels so disconnected to the real thing, at least to me. It doesn't come grieviously to rob you of your life and future: those are already forever gone by the start of the story. It doesn't come mercifully to relieve you of your pain and suffering: Mori already relieved himself of those at the start of the story. This second death is still scary and terrible, yes, but it's also robbed of much of the pathos of the real thing to my eyes, because everything has already been lost and gained. Even the possibility of framing Death as a friend, come to free a sick man of his suffering, doesn't work in this context because Mori has already gained freedom from it.
Again, there's this weird tension, I feel, between living Mori, on the one hand, who is someone who is supposed to have known for a long time that death was around the corner and I would expect to have a more mature outlook on death, and dead Mori, who is effectively a newborn robbed of his new life by surprise and who reacts to it as if it was something sudden and unexpected.
I hope that makes sense and again, I hope you don't take it too harshly. The story is still great! :-)