Is it permissible to marry a woman who has recently converted to Islam, despite the mother's strong refusal of this marriage, and is it permissible to conceal the marriage from her in this case?

Calling to Allah is a tremendous virtue. If a girl embraces Islam, and her Islam is good, and her situation becomes upright, and marriage to her is possible, then it is a good matter. You should strive to convince your mother to marry her, seeking assistance from influential people. If she agrees, then praise be to Allah. If she insists on refusing without a legitimate Shar'i reason, then it is permissible to marry her even without her consent, while striving to win her affection later on.

There is no sin in concealing the marriage from your mother, but informing her might be better. Weigh the two options and consult trustworthy individuals. If marrying her becomes easy, then good. Otherwise, cut off the relationship with her for fear of temptation, and you can then look for a righteous man to marry her.