Oh, this was so delightful! As an HP and a Holmes fan both, I absolutely wallowed in the way you brought two of my favorite flavors together. Watson's voice is impeccable, and I thought you did a bang-up job using his phlegmatic personality to make the concept of magic seem plausible in a 19th-century setting. It was also very sly of you to put the onus of moral huffery and adjustment on the poor doctor, re: the love that dare not speak its name. The fact that Holmes took it very much in stride, added to his familiarity with the male brothel, gave me a smirky moment to indulge my own private interpretation of the great detective's bent.
The plot and the pacing of this are marvelous, not an instant's excess or confusion. The scenes in London are wonderfully vivid, as are all the personalities involved. Your OCs blend in and genuinely enhance the story, and I enjoyed the glimpses into Prewett and Malfoy ancestry. Magic in the Victorian era is an irresistible concept, and I loved the way you handled the overlapping influences of time travel and cause-and-effect. So much invention here, so much to dazzle and fit together like well-chosen puzzle pieces.
The relationships among the five men are a constant source of entertainment, and my hat's off to you for juggling so many characters at once and getting them right. After Snape's revived and out of danger, the Snarry component provides the necessary emotional underpinning to the action, and you're very deft at weaving in the slow, smoldering build of reciprocal interest as a parallel to the brisk surface of a caper fic. I have a fondness for outsider POVs that bring their own unique take to the interactions between Snape and Potter, and Watson's decency, befuddlement, admiration, unease, and general alertness make him an excellent foil. I'm also glad that you gave us only as much romance as the doctor could bear, or it might have strained narrative (dis)belief. And I like that you make Harry and Draco behave in a more grown up and intelligent fashion than is their wont in canon; it's always refreshing when Harry, in particular, grows a brain.
Making Moriarty a half-blood wizard was a neat trick, and I adored how, over the course of the fic, you emphasize what a powerful wizard Snape is. We all expect it of Harry, but there's something particularly arresting when Snape's allowed to be magically bad-ass (especially in Harry's defense). Like slashpine, I love to see him break out his duelling skills and demonstrate how formidable he can be without it always revolving around potions. In a related vein, the quiet similarities between Snape and Holmes are beautifully handled, touched on without being irritating.
All of this, together with your wit and timing and a complex plot that never escapes your control, makes this a feast and a fic to be savored. Brava!