Is clinging to the niqab and refusing to work in a mixed-gender and usurious environment considered disobedience to parents? And is the girl sinful for refusing to marry a committed man due to her father's fears of manifestations of religiosity?

You are not obligated to obey your father by removing your niqab and going out to work if you believe that covering the face is obligatory. In fact, it is not permissible to obey them in this matter, as obedience is only in what is good and permissible. Your father also does not have the right to prevent his daughter from marrying a suitable man. If he does so, he is considered to be unjustly preventing her marriage (ʿāḍil), and she has the right to refer the matter to the Sharia judge. Nevertheless, you must honor and obey your parents in what is good, for the rights of parents are great. And whoever fears Allah and perseveres, then indeed, Allah does not allow to be lost the reward of those who do good.