(a) The Gemara (74b) says that the Chacham is Oker the Neder retroactively. Hence the Gemara says in Shevuos (28a) that if someone transgressed his Shevu'ah and was subsequently Matir the Shevu'ah he becomes Patur from Malkos or Korban as applicable, since it now transpires that there was in fact never a Shevu'ah. Conversely, in your example the fact that there was never a Neder will mean that the Kidushin took effect, and the woman is Chayav Misah. However if her husband was Meifer her Neder as opposed to Hataras Chacham, this would not affect the Neder retroactively, as a husband can only uproot the Neder from now onwards. In this case the woman would not be considered married at the time of the Znus and would be Patur.

(b) In this case the same rule would apply, meaning that the woman was in fact married and is considered an Almanah and Zekukah l'Yibum. Obviously in this case Hafarah is not possible (as the husband has already died), only Hataras Chacham.

Dov Freedman

You are right that this is a case of Hasra'as Safek, and practically she will be Patur, but she has transgressed a prohibition with a Chiyuv Misah. There is an opinion that Hasra'as Safek is Chayav (Makos 15b).