This is really very good. My two tiny critiques are: (1) I felt that it ended somewhat abruptly, although I liked the idea of Snape and Harry doing a ritual together; and (2) From about part 2 scene 2 to the end, it is absolutely RIVETING, but parts of part 1 felt a bit disjointed or like they needed trimming - it is not as compelling as part 2 scene 2 ff.
However I'm only mentioning these things because it's otherwise such an amazingly riveting fiction - really excellent job. thank you for writing this and sharing it with us!!!