Is it permissible to marry a recently converted Muslim woman without the consent of her non-Muslim family, who might kill her if they learn of her conversion, in order to protect her and enable her to practice the rituals of her religion?

There is no impediment to marrying a Muslim girl who has recently embraced Islam. In fact, you might even be rewarded for it, as this will strengthen her adherence to Islam and help her observe its rituals. It also protects her from perdition and from thevengeance of her family. The judge will officiate her marriage contract, because her father's guardianship is nullified by his disbelief. If she fears her family, then she should report her situation to the authorities in her country.