1) Perhaps take a look at "file" injection. This type of injection tells Fluent (among other particle properties) to read x, y and z positions from an external file. There is specific format to do better refer to the manual. A turn around could be to: during the mesh generation process, split your mesh into two fluid regions, one of course delimited by the initial height of the bed, while the other is simply the rest of your cells. This way Fluent can later recognize them as individual fluid regions. You can then use surface injection and choose the int_fluid region of interest. Particles will be injected from the interior faces of the cells defined by this region, as if they "sprout" from the mesh itself.
2) I am not sure about cohseive forces, but particle-particle interaction can be accounted by the DEM model.