Following up on yesterday’s discussion, we left off with the Ramban who holds that the issur of washing starts at the time of seudah hamafseket. As was pointed out in the comments, the gemara (Ta’anis 30) tells us that the issurim of washing, anointing, not wearing leather shoes, and not having relations are not only prohibited because of the issurim of ta’anis, but are prohibited based on hilchos aveilus – on 9 Av we adopt the halachic practices of actual mourning. This question, however, remains – why must we adopt the practices of mourning (at least with respect to washing) during the seudah hamafseket when the day of 9 Av does not begin until nightfall? M’mah nafshach - if one has accepted the fast from seudah hamafseket, then why is eating and drinking permitted until nightfall, and if one has not accepted the fast, why is washing prohibited?
One approach (see Keren Orah, Brisker Rav, and Chaim Markowitz’s comment from yesterday) is that the prohibition of aveilus from seudah hamafseket are not due to the acceptance of the fast, but are because dinei aveilus apply to the seudah itself. The Rambam (ta’aniyos 5:9) describes how the seudah hamafseket is eaten as if “one’s dead is before him” – what halacha would define as the status of onein. Although the RI”F and Rambam hold that an onein is not obligated in nihugei aveilus until after the deceased is buried, the Ramban disagrees, and holds that an onein must immediately practice the prohibitions of aveilus which will not interfere with his preparing the meis for burial. An onein is permitted to wear shoes to expedite movement to carry out the tasks needed to prepare for kevurah, but is not permitted (according to Ramban) to wash. Therefore, although eating and drinking are permitted until the fast begins, the seudah itself must be observed in a state of animus/aveilus, which prohibits meat/wine/washing.