As a possible alternative (as I haven't tested your issue of building on WS 2012), why don't you use TFS online? It allows for 5 free projects (privately hosted in the cloud). See more here: http://tfs.visualstudio.com/
More on your issue: I have tried running the WP8 SDK on Windows 7 without any luck (but it's not supported so I understand). Also, I have tried running the WP8 SDK from a Windows 8 Virtual Machine, but the emulator did not run as Hyper-V can't run on Hyper-V without pursuing this option: http://www.developer.nokia.com/Community/Wiki/Windows_Phone_8_SDK_on_a_Virtual_Machine_with_Working_Emulator
Hope this helps in some way, shape, or form.