A: No. The Xbox 360 demo can be run via Xenia emulator with heavy glitches, but no official PC demo exists.
A: Only via cloud gaming with third-party mapping software (like reWASD). Natively via streaming, no—an Xbox controller is mandatory.
Why? In 2014, the PC gaming push for Xbox was in its infancy. The "Universal Windows Platform" (UWP) had not yet matured. Consequently, you cannot find Forza Horizon 2 on the Microsoft Store, Steam, or any other digital PC storefront.