Is it permissible for a woman who wears the niqab to remove it while maintaining the lawful hijab, due to her family’s opposition to her wearing the niqab out of fear of persecution? And if she does not agree to remove it, is it permissible to marry her without the parents’ consent?

The niqab is what covers the face, and covering it is obligatory, especially in times of fitna (strife/temptation). It is permissible to uncover it if a woman fears harm will befall her because of it. As for marrying a girl who insists on wearing the niqab, disobeying one's parents becomes unlawful if their منع (prevention/prohibition) of the marriage is based on a valid reason that would result in harm to you, in which case their obedience is obligatory. However, if their منع (prevention/prohibition) stems from an objection to religious practices and virtuous morals, then you may marry her while striving to gain their satisfaction.