Towards the botton of 29b, the gemorah discusses whether Ma'amer is Nisuin or Arusin. It concludes it must be Nisuin because if it is Arusin, then how could R' Eliezer say that either husband could be mafir, when we learned in the Mishnah that a Na'arah who is an Arusa requires both the husband and the father, which shows the yavam only could not be mafir her nedarim. The gemorah answers that when he said mafir, he meant the yavam fulfills the husband's part of the hafara, but the father is still required.
I don;t get this. Who said R' Eliezer was discussing a case where the Yevamah was a Na'ara? It was never mentioned before! If the Yevama was a bogeres this answer would not work. Moreover, the yevamah is a widow. Even ih she was a na'arah, once she was married and became a widow doesn;t she leave her father's rishus so that his hafar would not be needed anyway?
In general, when the Beraisa mentions the Hafarah of a Neder of an Arusah, it refers to a Ne'arah ha'Me'urasah, particularly when this Mishnah was taught there in the discussion of a Ne'arah ha'Me'urasah (see, for example, the Mishnah in Nedarim 71a).
Regarding your question that the woman has already become a widow and thus her father should have no authority of Hafarah for her Nedarim, the Halachah is that a Ne'arah does not leave the Reshus of her father when she is widowed from Erusin, but only when she is widowed from Nisu'in. She remains in her father's Reshus when she is widowed from Erusin (regarding this situation, the Mishnah (Nedarim 70a) says, "Mes Ba'alah, Nisroknah Reshus l'Av" -- "If her husband dies, she is left in the Reshus of her father").
Mordecai Kornfeld