Is it permissible to offer expiation for a vow (kaffarat al-nadhr) regarding wearing the niqab after the questioner vowed to wear it if her marriage to the first suitor became easy, then the engagement was dissolved, and another man proposed to her, whom she did not stipulate the niqab to, fearing his rejection?

If you are certain that you made a vow regarding a fiancé whose engagement was broken, and you are unsure about the generality of the vow, then you are not required to fulfill the vow if marriage to that young man is not possible for you, because certainty is not removed by doubt. As for covering the face, we have preponderated its obligation. You are not required to wait until you find a suitor from a people who are accustomed to it. Rather, whenever you are able to adhere to it, do so. The niqab specifically is not obligatory; any method that achieves concealment is sufficient.