Well, it's taken me all day to read this, and I enjoyed every minute of it. ♥
I really like both your Snape and Harry here, and I especially like "portrait Regulus" - he's a wonderful addition to an interesting and exciting story.
I like the way you build the attraction between Snape and Harry - sometimes they just seem to "love" each other all of a sudden, but your development worked very well for me here.
Since the story is so long, there are many, many things I liked; it would take way too long to mention them all. They well and truly over-rode the two tiny things that made me blink, but I changed them as I read, so that was all right.
The main thing I liked was that this is a long, well written Snarry, that kept me interested and involved and impressed THE WHOLE WAY THROUGH. A first-class piece of story writing.