Is it permissible for a woman to visit graves, given the existence of a hadith that prohibits it, and if the hadith is authentic, does she incur a sin that necessitates expiation?

Women are not permitted to visit graves, as the Prophet (peace be upon him) cursed women who visit graves. It is obligatory for a woman to refrain from visiting them. If she visited them out of ignorance, there is no blame on her, but she must repent and not return. Visiting graves is specific to men for remembering the Hereafter and supplicating for the deceased. Women were prohibited from visiting graves due to the potential for fitna (temptation/strife), their lack of patience, and their excessive grief, and this is out of Allah's mercy towards them and towards men. However, there is no harm for women in performing the funeral prayer (salat al-janazah). There is no kaffarah (expiation) for one who visited graves, but only repentance.