A grab bag of impressions/stuff that I really liked about this upon first reading:
The idea of Harry-as-snake-charmer is incredibly witty, and something about the circus setting early on provides a sort of Something Wicked This Way Comes atmosphere and sets the stage for the entire piece somehow.
As I was reading, I was so incredibly frustrated with Harry not telling anyone in the mirror world what he'd experienced... but that's utterly in character for Harry, innit? He really is a bit secretive by nature, and often keeps things to himself. He does have some Slytherin leanings despite what JKR might say, and that's one way that it shows.
Crane is obviously Snape from the get-go, and I like that Harry spots it right away. Although Harry can be one-track minded and oblivious to things he doesn't want to see, he's not an idiot, and moron!Harry is an overused cliche`. But at the same time, Snape is more cunning, and you've allowed him to stay a few steps ahead, which is also great.
Your Snape here is intriguing. He's snarky and hard, his defenses are constantly up... yet through Harry, we can see that he's a man with a lot of insecurities who feels deeply.
I think my favorite part of the entire thing might be the banter between Harry, Draco, Remus, and Regulus. Especially the snark between Harry and Draco, and then Snape coming in and settling it with three words. Heh.
Oh, also? I like that, in the end, Harry does the "right" thing (more or less) without doing the foolishly heroic thing. It seems as though he learned a thing or two from his experiences during the war. Nice.