Oh, how I wish I could give this fic marks higher than 9's. You have just blown me away with this amazingly inventive crossover. I don't even know where to begin with the things I loved about this story.
Your world building was absolutely impeccable. I really lost myself in Victorian London, between the physical descriptions of hansom cabs and drays, gas-lights and carpet-bags, it truly transported me back in time. Watson's narration was brilliant, again reinforcing the sense of time through the choice of language used. Beyond that, the characterizations of Watson, Holmes, and our three modern day wizards was completely spot on for me. Draco's initial sneering disregard for everything and everyone Muggle felt completely right, yet we see the shift of perception begun in Deathly Hallows continue here. Harry displays competent yet understated perception of crucial elements, yet cluelessness of certain subtleties(the obscure password for the headquarters of the resistance), while continuing to show that Gryffindor impulsive valor coupled with mastery of defensive magic. Snape's incisive, skillful and deliberate actions and snarling defense mechanisms were all so fitting. As Harry has realized, he is a formidable wizard who can be relied on to do what is needed. Watson's demeanor and voice as well as Holmes' enigmatic and brilliant deductions also felt completely true to character, and were so reminiscent of Doyle's work.
The plot was intriguing and involved, and kept me glued to the screen. I loved the way that you pulled in ancestors, with the Prewetts showing the courage and sense of family noted in their descendants, as well as the thread of arrogant and cruel aristocrat connecting Lucius back to Tarquinius. Encantadora was a nice little twist, becoming the sister for whom they were searching, and Mrs. Calloway did seem a fitting predecessor to Bellatrix. Seeing the wizarding world and the relationships between Harry, Severus and Draco through the filter of Watson's world view and sensibilities was so fascinating and engaging.
The pacing was perfect, as you built both the elements of the search for the watch and the need to defeat Moriarty/Mortdelavie(brilliant!) at the same time as you progressed the relationships between the five characters. It was wonderful to see the masterful minds of Snape and Holmes working in tandem, and I loved the way that Watson gradually realized how much he had underestimated Harry. The "showdown" at Stonehenge was tense and suspenseful, and I was delighted to see Draco's Slytherin nature move past his anti-Muggle biases to use a revolver to defeat Moriarty. The way that you got Snape to finally reveal his feelings for Harry was perfect, as was Holmes' facilitation of that event.
I know there are many other elements of the story I'm neglecting to include here, as there were so many details that added to the compelling richness of this tale. Fantastic work - thanks so much for sharing it with us all.