Just a thought. If Harry used the time turner to change what happened, wouldn't he be the only one to remember what happened? If he changed the past they would only know the "corrected" timeline. That being said, I like the essence of your story (I just don't particularly care for diary stories. But that a personal issue, nothing to do with your writing.) I have developed a soft spot for Snap, and I have always liked Lupin. It's good to see them finding someone to care about.
Very interesting story! I think the decision to make it diary entries was a good -- lots of info can be included. You've kept me on the edge of my seat. Looking forward to more!