My suggestion is to adjust your GUI/Cameras to Viewport. Even windows mobile devices have different resolutions and aspect rations.
for example this line with scale to fill the viewport
var vp = GraphicsDevice.Viewport;
spriteBatch.Draw(titleScreen, new Rectangle(0, 0, vp.Width, vp.Height), Color.White);
Think of graphics.PreferredBackBufferWidth as input for ApplyChanges(), don't use it to get the current backbuffer dimensions, although usually it will have the same value.
You can set the window to 800x480 if you like but use the desktop project for that.
AppStore apps dont resize the window, instead they scale the backbuffer.
(also you skip the 'Deploy' step while you debug)
use this code in your Game() constructor, or in Initialize().
graphics.PreferredBackBufferWidth = 800;
graphics.PreferredBackBufferHeight = 600;