I want to question why Sakuya (or anyone besides the Shiki's) is part of this, but she fits it all too well. Wonder how interesting Tsukihime would have been with her as the maid of the house (or in addition to Kohaku and Hisui).
They're all knife/dagger-wielders if you're wondering. I was about to question Cheria's presence, actually. She's the real odd-one-out considering the series she's from. The Tales~ series isn't exactly the dark, more moody type series the other are more representative of. Then I saw all the dagger and remembered that they are Cheria's weapon-type. Though technically Cheria is more like Sakuya since they use throwing knives. The others use actual daggers in the more traditional (if supernatural) method.
Oh actually I just brought her up to make that comparison, but I like your explanation better.
Leaders of the Anonymous assassination squads? Well since they all have masks, even though they are not the same as anons, they still have them. Aside from the typical Venetian line, I want to believe they are part of Anonymous.
Actually Stahn, Cheria can stop time as well, considering her Accel Mode and her 3rd Blast Caliber. She really has some of Sakuya's blood.