I am posting reviews somewhat out of order.  I neglected, with the chapter 7 review, to mention your rhymes and clues.  They are very clever and you get a lot of credit for devising them. 

Where you write: "Thank you Rosmerta, please lock the door behind us and then call the Auror's headquarters and talk to Colin Creevey or Hannah Abbott."  Rather than "call," should not be, e.g., send an owl.

"As Harry's hand closed on the doorknob a hazy purple mist began to swirl around their feet." – I was really pleased to see a magical element make its appearance.

The communique from the killer: probably would be a good idea to italicize it, or use blockquotes, or use some other means to set it apart from the rest of the narrative.

"What was the first name of the first man you m-m-murdered?" – I question your use of the word "murdered" here (with reference to Quirrell), although you can make a case that Corner so despises Harry that he refuses to cut Harry any moral slack and therefore subjectively sees any killing by Harry as a malicious, unjustified killing – a murder. But I think that theory is pushing it. Corner may be crazy as a bedbug, but he’s still a former DA member. I tend to think he would recognize Quirrell’s killing as an act of self defense – a killing, rather than a murder.

I'm also impressed by the swift pace of this story; and this observation pertains not just to this chapter, but to the story as a whole.

I think you did a fine job of writing this final chapter.   

I do  have to say, however, that Michael has all my sympathy in the final scene.  No, there's no excuse for killing innocent people.  But it appears that Michael was betrayed by Harry and his cohorts, who chose not to rescue him, leaving him and Lavender to the tortures you described so graphically.   The US Marine Corp never leaves a man behind -- surely Harry Potter and the DA should be up to the same standard.  

So, sorry to start off on a negative note -- because you have a fine melodrama/procedural going (as I've mentioned before).   Because his extenuating (mitigating) circumstances are so strong (months of torture, possible madness, chronic unbearable pain, witnessing the torture of a comrade/loved one), I'm not sure Michael would end up in Azkaban -- or, certainly not doing "life" there.  This seems like the sort of case the Ministry would be very anxious to plea bargain, in order to keep the lid on those facts that reflected very badly on their hero. 

I think you have a lot of really great elements going on with this story.  At some point, you might want to play with the structure.  (For example, I'd love to see Draco have a bigger role, because he has potential to be an even bigger red herring.  After all, the kid was found with a potions book; he has a sort of motive; and he's a smart***.)  

I would also like to see more magic.  As these people exist in a magical environment, magic would be part of their crime, and it would be odd for them to employ a "muggle" spell.  In other words, the cipher needs a spell to go with it.  Otherwise, it's "Law and Order:  MVU."   

This is a very ambitious series, very cleverly plotted.  The plotting kind of took my attention from the descriptions, which were very good.

Congratulations on this project and on bringing it to a logical conclusion. 

This chapter had a good opening and dynamite ending.  I also liked the brief section devoted to Cho's POV.

A couple of things bothered me. 

Where Harry says, "Michael., can you go see what you can dig up about this girl? I need to know everything. . . Get me a report as soon as you can." – all fair questions, but wouldn’t he want to start by asking Mme. Pince what the kid was doing in the Restricted Section? And then sending Michael off to interview the professor who wrote the note that got her in there? And the last people to see her?  And then, I  wondered that he started in with these very questions of Ron, who would not be expected to have such knowledge. (Perhaps a more natural question to Ron might be along the lines of, "what do you know about this?")

I understand that Draco and Harry are not one another's favorite people.  But Harry is supposed to be professional and Draco has restraint and breeding; and they both have maturity.  If you have Harry hit a suspect, then threaten him with Azkaban (doesn't Draco have some sort of legal counsel to go with all his money?)  -- well, that sort of behavior may be fine with some of your readers, but his tactics just lost him about 10,000 point with me in the respect department.  

If you will forgive me saying a little more on this point:  good detectives are clever, psycholgically astute questioners.  In this particular case, sparring between Draco and Harry in the respective position you've placed them could be great.  I think that verbally Harry is being too heavy-handed and Draco too submissive (Draco stood up to interrogation by Snape; he's good).    If you ever consider expanding the scene where Harry questions Draco (obviously, I'm not crazy about it as is because of the coersive aspect, but that's another issue), you could have a lot of fun with it.

And I have to comment that we are at Hogwarts, and there is still no apparent use of magic.  And that's fine.  You are more than welcome to tell me in response to clam up. 

The ending cliff-hanger -- again, well done.  And I like that Hannah and Tonks are now part of the picture.

I forgot to mention the cliffhanger ending in the previous chapter. That was very effective and made me have to jump to the next chapter.

The anagram confused me, as there were extra letters. That seems like a slightly different kind of code where she would first have to eliminate extraneous letters and then rearrange the remaining ones. A suggestion, because this still resembles a muggle procedural (and because Harry is puzzled) is for her to demonstrate by rearranging the letters magically, the way Tom Riddle rearranges the letters of his name in COS (the movie, anyway).

Concrit: The lack of magic in the code is suggestive. It seems odd that there is no spell to break. It either points to a very unskilled witch/wizard (cough Umbridge??? cough) or a muggle. The absence of magic is such a strong clue that I would expect someone to remark on it.

"Harry had murdered the dark lord" – should be, "Harry had killed the dark lord . . .".

"Murdering" someone is different from "killing" them. By contrast, poor Cedric was murdered.

The final puzzle and the murder requiring a trip to Hogsmeade made a neat ending.

Well written chapter, with an interesting AU set-up.  I think you have started a really nice procedural. (I like that Colin is a crime photographer -- it fits him.) 

My only real criticism is that, so far, this appears to be a "Muggle" murder mystery (yes, I know the killer makes reference to the Dark Lord, but we muggles use that expression, too -- to refer, e.g. to the devil).  At this point, I think the story would benefit from more magical elements about the crime.