What a creative and intriguing fic! I've read a few Harry-and-Severus-in-the-afterlife fics, and yours would be a good fic just in that area with the neat details of the cloud-watching/people-watching and the increasing closeness between Harry and Snape (and the great drunken sex scene). But I think you really added something to the category with the parallel relationship/realizations/moments in the re-lived life once Harry and Severus return to earth and experience a what-should-have-been outcome, in which Harry does not die, Severus and he develop this intimate relationship through letters, and Harry's family is not as shaken by the various betrayals of Harry's marriage. I wasn't sure how the re-casting would work, but the two-parts of the story played out so well in the themes and parallel developments of Snape and Harry's love.
All the little echoing details were great -- the hazelnut coffee changing to the hazelnut potion, the visits with Albus in heaven paralleling the visit with Evander in the re-lived life -- there are a bunch I know I'm not naming, so I think you threaded the details through well there.
One hesitation I had was how much the outcome of the wrongly-lived-life set Ginny up as a source of evil, literally if she's the mother of the Queen of Evil. That seemed a harsh punishment for her infidelity, and it was hard not to read it that way, as a comeuppance for her betrayal of Harry (though at first I thought maybe she started the affair when Harry was very ill -- still wrong, but far more comprehensible and pitiful).
But on the other hand, her inadvertent poisoning of Harry through her own lovesickness and unawareness of the dangers of the mail-order potion in the re-lived life seemed to me very realistic and sad, in that Ginny thought she wasn't harming Harry but helping him on his quest (if they were both truly in love, that's what it would have done, if I'm understanding correctly).
It was great to see Harry get some resolution with Dumbledore, and good to see an outcome in which Ginny's relationships with the children weren't tarnished by their discovery of her cheating. And the idea of Harry and Severus writing all those notes and letters, building their affection and hidden desire that way, each of them saving them and separately yearning for each other, very persuasive and touching. I loved the epilogue, with them at last truly living the life together they were meant to have, with Harry's children fully accepting Severus into their clan.
Definitely a terrific read -- once I got into it, I just wanted more, so I'm very happy you ended up writing everyone such a long Snarry!