This is not a signed-in review because my stupid computer is doing weird stuff, but that was a very creative way for Harry to get out of his predicament.  And I think you are pretty accurate in your portrayal of how Sirius would have reacted in the situation.  It's a little contrived that he had to stay in the house, but it works.  Even without that, I think the Sirius we see in OotP would have had trouble being a good guardian for Harry.

The scene where Harry breaks-down brought tears to my eyes.  Having Snape finally gently chide him kept Snape very much in character for your story. 

I really like this chapter.  In the long run, the description of Harry living with Sirius is probably realistic.  If Sirius hadn't died at the end of OOTP, and if Harry had been able to live with him, a similar scenario might have developed.  Sirius bitter, drinking too much, having difficulties with Harry not actually being like James, not really parenting Harry very well...  I like the way you had Harry dream Snape's visit, clearly showing how Harry appreciated Snape's actions and care for him.

That was a perfect way of having Harry come to see reality better.  And I like Snape's response to Harry when he "lost it" in his anger, frustration, and grief.  Excellent chapter.

I'm curious about where you're going.  I assumed at first that you were just working through the summer.  But now I'm wondering if you'll keep this going into the next school year.  It's really developed very well and could go on at length if you wanted.