This is very well written. I like the intensity as well as the extremely believable dialog. The only concern I have is the use of present tense. You're doing quite well with it, but present tense can be very difficult to sustain over a long period.
Your Snape and Harry are both much in character. I loved Harry's reasoning behind why he doesn't want to know whether or not Dumbledore really knew about what the Dursleys were really like. And I very much hope that Snape is right about Dumbledore. Everyone has a different "take" on what Dumbledore is like. Actually, I think he's more like you have Snape describe him. And if anyone should know, it would be Snape.
Author's Response: Thank you so much. You're not wrong about everyone having a different take on Dumbledore---but I have very firm opinions about what I think he's like and what he's capable of. And, not to give too much away, but I usually stick my opinions in Snape's mouth, so make of that what you will. :-)
Incidentally, I write in the present tense so much that nowadays I find it almost impossible to write past tense---I keep slipping back into present without noticing. :)