1) Phobos is in a low orbit and closer to the orbit of mars surface-imaging satellites, so it is relatively simple to rotate a craft to take an image of it (Diemos is in a more distant orbit, accessible only during the hectic and worrying phase of orbital capture or circularisation when there are other more important things to worry about)

2) Phobos is about twice the diameter, nearly 4 times the surface area, and 6 times the mass of Diemos

3) It is pure science fiction to talk about causing a moon to crash into its primary. We won't have that ability for many centuries, if ever.

Other than that, and a few more things, your question has merit. I's like to know more about Diemos...