Is a single woman obligated to fulfill her promise to her deceased father to wear an "isdal" over her face, even though this is undesirable for a single woman?

The covering of the face and hands in front of non-mahram men is obligatory according to the most preponderant opinion of scholars. Your statement that the niqab is undesirable for a single woman, if it is due to custom, then it holds no weight as it contradicts the Shari'ah. If you mean that it is religiously undesirable, then that is a statement against Allah without knowledge, and it is among the gravest prohibitions. Therefore, it is obligatory for you to wear the niqab in obedience to Allah and to your father, and in fulfillment of your promise. Complete hijab is a sign of chastity.