Is it permissible to refrain from marriage in fulfillment of a vow to a deceased husband and in aspiration of meeting him in Paradise, and is there a reward for such fidelity? Also, is wearing the niqab, despite parental disapproval, considered a dereliction of duty towards the daughter if it affects employment opportunities?

If you are safe from temptation (fitna), there is no harm in abstaining from marriage out of a desire to accompany your husband in Paradise, or to devote yourself to raising your daughter. For this, you will receive a great reward. You may be like the woman referred to in the hadith: "I and a woman with dark cheeks on the Day of Resurrection will be like these two," (and he pointed with his middle and index fingers). And your loyalty to your husband is commendable. As for the niqab, scholars have differed regarding its obligation. It is preferable to cover the face due to the protection and modesty it provides. However, if it causes definite harm, it is permissible to remove it, limiting your outdoor excursions to necessities.