First a little lomdus, then a little machshava:
1) When 9 Av is a nidche, the halacha is that a ba'al bris does not have to finish the fast because it is his Y"T.
You still have to fast even though it is a nidche. Why should a ba'al bris have a different halacha than on a regular 9 Av?
Rav Wahrman (Sheiris Yosef vol 5) quotes from R' Leizer Silver and the Rogatchover that a ta'anis tzibur is not just a chiyuv on the gavra not to eat, but there is a chalos in the day.
It could be that a fast which is nidche does not have that chalos in the day. It is not 9 Av on Sunday -- it's 10 Av. There is still an obligation to fast, but that is a chovas ha'gavra, not a result of the status of the day itself. Therefore the Y"T of the individual can override it.
He has a number of proofs, but on to other things...
2) Vi'varech eschem ka'asher dibeir lachem…(1:11) Chazal tell us as a general rule that dibur = kashos, harsh words, as opposed to amira = pleasant talk. Why then does the Torah use the expression "dibeir" when it is speaking about brachos given to Klal Yisrael?
It's Shabbos Chazon; it's 9 Av. We are at the climax of the season of kashos, or difficulties and harshness. What the Torah is telling us, explains the Tiferes Banim (the son of the Bnei Yisaschar), is that even when we are faced with dibur=kashos, there is always a "vi'vareich eschem" behind the scenes. Even when we are getting smacked, it is a bracha in disguise.
The gemara always uses the expression ta shema, come and hear; the Zohar uses the expression ta chazi, come and see. When you are learning pnimiyus haTorah, you have to look more deeply inside (inside the ideas, but inside yourself as well). Shabbos reveals the pnimiyus which is hidden the rest of the week. Shabbos Chazon, ta chazi, is about seeing the pnimiyus of the tzaros of churban; it's about looking forward to the day when we can understand how there can be an element of bracha even there.
The Shulchan Aruch gives us a siman to remember that 9 Av always falls out the same day as the first day of Pesach: aleph-taf. Aleph, the first day of Pesach, always corresponds to taf, Tisha b'Av.
R' Leibele Eiger (Toras Emes last piece in Devarim) writes that this is not simply a mnemonic, but it tells us that there is a relationship between these two days. At the moment of geulah from Mitzrayim, the seed of churban was already planted -- the aleph, the beginning, contained within in the end, taf. But by the same token, the end, Tisha b'Av, contains within it the seed of future geulah, a new beginning.
The gemara (Ta'anis 29) writes that when 9 Av falls on Shabbos one can eat even a meal like that of Shlomo ha'Melech. Why does the gemara use that expression -- why not just as "a big meal?" R' Leibele Eiger explains that the gemara is telling us that Shabbos reveals to us a little of the pnimiyus of 9 Av. Shlomo built the Beis haMikdash, fulfilling the dream of "zeh K-li v'anveihu" (see Targum Onkelus there) that we had at Yam Suf. When 9 Av is on Shabbos we experience a little taste of that fulfillment of geulah that ordinarily lies hidden within the day. We can celebrate as if we lived in Shlomo's times, with a Beis haMikdash, with hashra'as haShechina.
Hopefully we will see it b'poel in our time.