Go walk in. Turn my disguise up to 11, this time it's okay to look inhumanly beautiful. Or whatever beyond that is. Then move in, hang up the phone, and sit on his desk. Tell him to stop being an idiot, and to think about things before calling the cops. Maybe this is some kind of odd joke. Maybe she's innocent, and calling the cops will ruin her life for something that didn't actually happen. Maybe the key drive doesn't contain what the note says it does.
If it's all true, however, he's got a rare opportunity here. Tell him I'm a figment of his imagination, and keep the disguise shifting from one thing to another, along with some playing with the light projections to convince him. Talk him into not scotching the plan here and now.. after all, he can always sit on the money and wait, can always check if the money manager was good at covering her tracks before cashing in, lots of ways to take this carefully and play it safe. Tell him to just tell the cops that he lost his cellphone or something and called them to report it got stolen. He can always report this later, if he is worried about getting caught, but a chance this big shouldn't be rejected right away. Leave the office in a flash of light once I'm convinced he's settled enough, and before the cops show up. If the cops do arrive while I'm in here, wave the stone and tell the cops that they can't see or hear me. They might not even show up, and just phone back or something.. and if he'll talk them out of it over the phone, so much the better.
Assuming that goes well, leave and then call Dave and tell him I'm ready to put up more content, but I've got a few errands to run first. Ask him to come up with a design for an OC.. a non-human creature who Nanami works for. Then clean up the loose ends with the financial manager, by packing up some belongings, killing her immediate family, and going down to a train station and withdrawing the daily maxium of cash while disguised as her. Killing her family might be tricky, if she had some magical blood one or more of them might be casters, so sneak up, disguise myself, and shoot first. After that, ditch the evidence and head over to Dave's.