So, Reimu's the fake? Excellent.
A couple chapters back I commented it was a story about depression. I was thinking, of course, of Marisa's symptoms, which were reminiscent of burnout. (Both actual, mental burnout, and metaphorical in the way she lost her magic.) Now it turns out it applies just as much to Reimu. Losing Marisa dropped her into an abyss, but to the outside, she still had to pretend she was okay. She couldn't disappoint expectations - the Hakurei does not waver. Better to isolate herself completely until, some way or the other, she would be forgotten, and nobody would expect anything of her anymore.
Except Marisa, having been down in the hole, knows the way out, and she isn't going to let go of Reimu's hand this time. And that's the story of true friendship... which is indistinguishable from love, really. It's just one facet of it.
The story hews very close to canon... mostly. This interpretation of Reimu is much more stoic and melancholic than the Reimu we usually see. ZUN writes Reimu doesn't experience character development - some may have taken that to mean that she's stuck forever being someone, or rather something, that she didn't choose to be, and that's why she's so grumpy - she is unhappy, deep down. I take a more optimistic stance - Reimu won't experience character development because she is legitimately content, happy, being who she is, deep down - but I deeply appreciate the counterpoint offered by this work that just because someone looks like they're content - "She'll be okay" - it doesn't mean they are. Not by a long shot.
I'd love to know if there's any word on what's next for the author. Do they have another one in them? I'd love to see it.