Does the case fall, or is the decision taken by the majority of the heirs, or does the right of the relinquishing heir alone fall, if one of the heirs relinquishes their mother's right in a case in court?

Heirs are partners in financial rights, and each heir has their specific share, which they can only waive for themselves. If one of them waives their share, it does not nullify the right of the others. As for the right of retribution (Qisas), if one of the heirs waives it, the entire Qisas is forfeited, because it is indivisible. Any heir who does not waive their right will receive their share of the blood money (Diyya).