I got the feeling while reading this that the writing of this story may have been cathartic for you. The first bit seemed to be focused on tieing up many of the loose relationship ends left by JKR, while the second half felt like a prelude to a shared twilight of existence. I'm glad that you left the ends of their lives ambiguous, but accepted: I'd have hated to think of them never finding peace after happiness.
An interesting story. I have to admit, I liked the last half of it much better than the first, so I'm glad that I stuck with it. But I trust you as a writer, so I was sure that there was going to be a payoff. By the time it came, it seemed part of a natural progression - very nicely done.
It's just weird. I can't seem to shake the feeling that this story reads like a farewell. I'm curious to see where you plan to go next.