ok. first of all i would not bother trying to include reflections in a shadow pass - leave reflections for their own pass (described at the bottom).
for the shadow pass i've run into repeated limitations with maya's useBackground shader, so now use the white shader method. basically just make a white lambert or blinn, set diffuse and color to 1, and everything else to zero. this is much better than a useBackground shader because you can map the transparency of the shadow material and it will actually work 
doing a seperate shadow pass is always a good idea - if you need more contrast or bright areas in the render just increase the diffuse above 1 slightly, or even just the color and contrast in post 
this pass is set to Multiply in post. you may find that you color pass from maya is a bit dark, i prefer to render my color pass with increased ambience on all materials to lift the image up a bit, just to a level where things aren't washing out. you can always control contrast in post but if things start off too dark and there isn't enough information in the image to begin with then your screwed.
also... always render shadow passes with raytracing in mental ray. what mental ray lacks in shader functionality for color passes, it makes up for with brilliant shadows, which are much faster and vastly superior quality than maya's. you can also use mental ray shadows for the specular pass, just set lights to "emit specular only".
for reflection passes i would just do a normal maya render but set the matte opacity of all objects to 0, except the objects which contain reflection (leave em at 1). this will render everything in RGB, but give you an alpha containing just the reflective objects. i am still currently using maya raytracing for reflection passes, although it renders very slow, but mental ray shaders are not very functional in maya 4.5.... so not much choice.