The middle part of the mouse was fairly simple to model, firstly I made a sketch of the side profile of the mouse shown here in blue (sorry about the wonky line, i did it with my trackpad).

This profile was then mirrored around so that it made the shape of the top part. I then use a projected curve using the profile from the bottom part and the profile which was shown in blue. I then had to use guide curves to create the proper shape. And  finally i used the surface loft tool to create the middle part of the mouse (highlighted).