Is it obligatory to obey a mother who prevents her son from congregational prayer because she denies its obligation, while disobedience would result in harm? And is it permissible to pray individually if the family members refuse to pray in congregation or prevent prayer by force?

Obedience to parents is obligatory except in disobedience to Allah. So, if a mother prevents her son from congregational prayer – which is obligatory – absolutely, then there is no obedience to her, because it is an obligation, and there is no obedience to anyone in disobedience to Allah. However, if she prevents him from praying in the mosque on condition that he prays in congregation at home, then there is no problem in obeying her. Likewise, there is no obedience to her if she prevents him from attending congregations permanently.

The son should strive to convince his mother of the virtue of congregational prayer and the necessity of observing it. If she insists on preventing him and he fears harm from her, then he is excused. He must be dutiful and kind to her, and advise her with good manners and gentleness.