1: Yes, you could petition your deceased grandfather, though he wouldn’t exactly fit the definition of a “higher power.” Ancestors are often petitioned for things like protection as they usually have a vested interest in your well being.
2: I don’t think such a petition would be acceptable. In my opinion, you would have far more success petitioning Lucifer to help rid yourself of your self loathing and desire for suicide. You need to keep in mind what spirit you are petitioning. Lucifer is the Light Bringer, and his purpose is Illumination and Knowledge. Suicide is not something he would condone.