Can one expiate an oath to marry only a niqabi, when this becomes difficult, but there is a religious and moral girl who wears the legitimate hijab?

There is no objection to marrying a girl who does not wear the niqab. Breaking the oath and offering expiation for it is better in this situation because the greater good lies in breaking the oath, and it is commendable to do so if it is better than persisting in the oath. Despite the jurisprudential disagreement on the permissibility of a woman uncovering her face when there is no fear of temptation, the wife must obey her husband if he commands her to cover her face from non-mahram men. The husband can also encourage his wife to wear the niqab after marriage.