What is the ruling on placing the Mushaf with the deceased in the grave for companionship?

What you did by burying the Quran with your mother has no basis in Islamic law. It exposes the Quran to what is impermissible. You have done well in your eagerness to show filial piety to her after her death. The best acts of piety towards her are supplication, seeking forgiveness for her, and other such righteous deeds.