This story is a seamless fusion of canon and new elements, and is elegantly told. You've painted a perfect picture of Snape as a man who is utterly, resolutely committed to doing whatever he must, no matter the cost to himself. And you show, in all it's painful detail, exactly what it does cost him. This line:
Sentimentality might've otherwise been useless to him, but in his past, he found purpose for the present.
Just so much yes.
Lovely that in the end, he's managed to gain back at least a portion of what he's lost -- and perhaps has even more to gain still. A much more satisfying epilogue for him, I'd say.
Also, I have to make note of this little stroke of dark comedic genius:
"What a lovely night! The skies are so clear I can see nearly all the way down to the school grounds. It'd be quite a fall from up here."
Indeed. Your characterization of Albus is perfect.