A careful reading of the Ramban seems to belie the understanding that bathing is prohibited from the time of seudah hamafeseket because the seudah is a quasi- state of aninus. Recall that the Ramban explains that bathing, as opposed to wearing shoes, is prohibited from seudah hamafseket because the enjoyment of feeling refreshed extends into 9 Av itself, not just at the time of washing. Why does the Ramban need this sevara if the seudah obligates nihugei aveilus? I think the answer to this question is that the Ramban is not offering a new sevara to explain the reason for the issur, but is simply offering an indicator (siman, not sibah) why only the aveilus pratice of washing was prohibited but not wearing shoes, etc. However, in Moadim u’Zmanim, R’ Shternbruch takes this sevara of the Ramban at face value as the reason for the issur – it is the hana’ah which comes afterwards, on 9 Av, which creates the issur of bathing beforehand. This too suggests R” Shternbruch, is the reason for the prohibition of wine and meat – not because of aninus, but because these foods provide relish and enjoyment long after the meal has ended.
The Rama paskens that on erev 9 Av one should learn only material dealing with bad events or aveilus which are also permitted on 9 Av. The GR”A describes this as a chumra yeseira, an excessive prohibition, and the Aruch haShulchan as well does not seem to accept this Rama. Wwhen someone accused R’ Chaim Brisker of being too machmir, he offered a list of kulos he held of, among them the leniency to learn anything one wishes until nightfall on erev 9 Av. R’ Shternbruch writes that based on the Ramban, we can perhaps understand this Rama. Pekudei Hashem yesharim, mesamchei lev – we cannot learn Torah on 9 Av because there is no greater joy than engaging on Talmud Torah. Just as one cannot bathe, eat meat or drink wine on erev 9 Av because that enjoyment extends into the ta’anis itself, so too, the talmud torah of erev 9 Av would undoubtedly fill one with joy even after nightfall. The difficulty of this approach is why these issurim do not apply to Shabbos when 9 Av follows on Sunday. Regardless of whether the Moadim U’Zmanin’s sevara is correct in lomdus, it certainly is a powerful mussar to take away from hilchos tisha b’av.