I'm not sure if I'll be participating much beyond this comment. This story is, in my view, completely perfect. It's perfectly written, I don't actually agree with you that Justin is "different" in voice or character from canon in anything but very minor ways (I'd argue that BRIAN is the one who is different, not Justin, although even for Brian, he's a Brian we know from canon, just not the most familiar Brian), and it's a wonderful piece of work.
I don't enjoy it because I don't like melancholy, sad stories. They upset me on a personal level. But to deny the power and perfection of this work would be something I could never do. I am just not one of those people who enjoys "delicious pain."
The fact that they are together at the end doesn't change what you call a "sense of trepidation" that the fic creates. This is a positive from a writer's perspective... she infused this story with so much emotion. It's brilliant.
However, I've read this story, not just the first time when I didn't know any better, but recently, as I'm writing a reunion fic of my own, as a sort of personal challenge. So it's possible I'll participate in this discussion. I have to see how it makes my hormonal upheaval me feel.