If a woman made a vow and violated it, and it was later annulled by her father/husband, can the court now annul the vow retroactively if it finds an opening, thereby exempting her from being liable?
It seems from the plain language of the Rambam in Haflaah, Nedarim, 12:19 that she would be liable regardless, and that a court can not help her here. The commentaries seem silent regarding this question too, suggesting the same.
However, see the commentary from חיל המלך on Haflaah, Shvuos, 6:18. It's not a super tough read, so I suggest checking it out yourself, but I'll write out my cursory read just in case it helps:
The Rambam over there sets the precedent that in a case where one violated a shvuas bitui (promise dictating behavior in the future) and then asked for release from the vow from a court, that would be effective retroactively. The חיל המלך brings out two relevant points to this question. First, the Rambam believes that even in a case where the original vow can no longer incur a penalty, such as if someone promised not to eat a specific loaf of bread, ate the whole thing, and then asked for release from a court, that release is effective. That seems a lot like our case, where the woman can no longer incur a penalty anyways because of her husband's cancellation.
The second point he makes is that this appears to only apply if the judgement would actually be carried out. Therefore, nowadays, since penalties of lashes and sacrifices would not actually be applied, this is not effective. He ends off by saying that we should do more research on the opinion of the Rambam on this final matter.
If you want to do more research, I'd check out the gemarah he quotes, then look for נר משפט on that page. If that doesn't make it into Shulchan Aruch because it discusses sacrifices, Likkutei Halachos from the Chofetz Chaim probably does rule on it.