There's something about the point where Severus looks at his reflection in the water and tells her "you're dead" that makes the whole thing make sense to me.
Not that I didn't believe it before, but you know how you suspend the disbelief for the sake of the story (and it was a truly amazing story, to be sure), but then at the end you almost want to keep your disclaimer.
"So, let's just say that this is what happens. Give it the benefit of the doubt and..."
Well, when he looks down into the water and tells Ginny that she's dead, that's what did it for me. It was such a Snape-like thing to do, such a natural thing, that I can believe the rest of what happens. Agreeing to marry Harry, having children, pulling off friendship with Hermione and being a Weasley.
And from that, my mind takes me to the realization that Snape will finally have the things he never had as a child. He now has a loving family, protectors, a place in life, friends, love, and hope.
I don't think it needs a sequel. The magic is in the fact that we don't know, and we can choose to simply accept it based on what we know of these characters.