First, a very simple question: The gemara clearly assumes that the hafara of the father OR husband alone accomplishes something, the only question is whether it accomplishes weakening the entire neder or removing half the neder. How then does the Ran (67a) explain that hafara of the husband or father alone is "very weak because by itself it is nothing", to justify why the hakama of the other can destroy the hafara of the first?
I thought it would be worthwhile to present the 5 cases of nisrokna to possibly provide some clarity in this complitacted sugya. Basically we always say "nisrokna" to pour the husbands rights into the father, but not the fathers rights into the husband, except for case #5 where the neder remains weakened so beis hillel maintains that we don't allow nisrokna even into the father's reshus.
1. Husband was not mekayem then neder (he was not actively mekayem and he either died on the day he heard about the neder or if he died the next day he was meifer on the earlier day), with the death of the husband, his rights of hafara are poured into the father who now has full rights of hafara. But, if the husband was actually mekayem the neder or mekayem it by default (he died the next day without being meifer), since the husband himself has forfeited his hafara rights, the father cannot inherit hafara rights from the husband.
2. Father was meifer and then died, the father's hafara is nullified with his death, and the fathers hafara rights are not poured into the husband. This is even if the husband did not hear about the neder while the father was alive which would give the father stronger jurisdiction over the neder, so that when the father would die there would be very little strength left to the neder, even that small remaining amount cannot be removed by the husband.
3. Husband was meifer and then died, the husband's hafara is nullified with his death, and the husbands hafara rights are poured into the father to allow the father to be meifer. This is even true if the father has not heard about the neder so that the husbands had strong jurisdiction over the neder and weakened to a degree where it is more difficult to pass on to the father, nevertheless the rights are still given to the father since with the death of the husband his hafara is nullified so that the father is actually inheriting a significant amount of the neder rights.
4. Husband is meifer and then the father dies, the husband cannot be meifer the portion of the father. This is true even though the father did not know about the neder so that the husband had strong jurisdiction over the neder originally, so that his original hafara removes most of the neder and there is only a small part remaining, even that small amount he cannot be meifer since a husband can never be meifer without active participation from the father.
5. Father is meifer and then the husband dies, the father can be meifer the portion of the husband. The opinion that the father can in fact be meifer is that of beis shamai, and this is true even if the husband did not know about the neder so that the original hafara of the father removed the bulk of the neder, we do not consider the neder "weakened" to prevent the father from inheriting full hafara rights and nullifying the neder by himself. Beis hillel disagrees and maintains that since the original hafara of the father was strong (because the husband didn't know about the existence of the neder) we consider the neder weakened to a point that the husbands portion on which there was no hafara done yet, is not transferable to the father and therefore the father cannot do hafara on such a neder.