Is it obligatory to wear the niqab immediately despite the mother's opposition, who considers it a virtue, not an obligation, especially before marriage and before sitting with the suitor while wearing the niqab?

You must be gentle and kind to your mother, but obedience to Allah comes first. The niqab is obligatory, and you must wear it in front of those to whom it is unlawful to look at you, while striving to please your mother and be gentle with her. You can seek help from righteous people or let her listen to lectures by preachers on the topic.