days of teshuva or days of mourning

1) Chasam Sofer (208) points out that according to Rambam (Hil Taanis ch 5) the fasts of 10 Teves, 17 Tamuz, 9 Av, etc are days of teshuvah, not days to commemorate the past:

יש שם ימים שכל ישראל מתענים בהם מפני הצרות שאירעו בהן כדי לעורר הלבבות לפתוח דרכי התשובה ויהיה זה זכרון למעשינו הרעים ומעשה אבותינו שהיה כמעשינו עתה עד שגרם להם ולנו אותן הצרות. שבזכרון דברים אלו נשוב להיטיב שנאמר והתודו את עונם ואת עון אבותם וגו':

Achronim point out that other Rishonim disagree, e.g. the Ran (end of 4th perek of Taanis) writes that there is a chiyuv to say neila on a taanis geshem but not on these days stems because taanis geshem is a day of tefilah and teshuvah, while these are days to mourn over the past.

Rav Shternbruch points out what seems to be an inconsistency within the Rambam, as in Hil Tefilah (13:18) Rambam distinguishes between the fasts that commemorate the churban, שאנו מתענין על מה שאירע לאבותינו, and other fast which are days of teshuvah:

 ובשאר התעניות שאנו מתענין על מה שאירע לאבותינו קורין בשחרית ומנחה. הראשון קורא ויחל משה ארבע פסוקים. וקורא השני והשלישי מפסל לך עד אשר אני עושה עמך. ובתעניות שגוזרין אותן הצבור מפני הצרות כגון בצורת ודבר וכיוצא בהן קורין ברכות וקללות כדי שישובו העם ויכנע לבבם כשישמעו אותם 

So even according to shitas haRambam, there is an element of commemoration, not just teshuvah and tefilah. 

R' Yitzchak Sorotzkin reads Ramban is a middle ground between these positions.  Ramban writes in Toras haAdam that the original takanas nevi'im that applied in eis tzarah was for the fasts of 10 Teves, 17 Tamuz, etc. to all have the same halachos of 9 Av.  Nowadays, when there is no eis tzarah, we continue to observe the fasts because Klal Yisrael accepted upon ourselves to do so, but we only accepted the din of not eating on the day of the fast, not the other chumros.  Why the split?  The takana of the neviim was to fast in order to commorate the churban, and therefore 10 Teves, 17 Tamuz were all like 9 Av.  Nowadays, we fast to inspire ourselves to tefilah and teshuvah. The different mechayeiv creates a different set of halachos.  

The gemara (Taanos 16) mentions a minhag to visit the cemetery on a taanis geshamim. Tos extends the minhag to apply to 9 Av.  R' Shternbruch points out that the chiddush of Tos may hinge on the above views.  One could argue that taanis geshem is about eliciting teshuvah, while the focus of 9 Av is mourning, and therefore the halachos are not the same (See Ah"S  457:18 who writes that yeri'im do not go to the cemetary, but for a different reason.)

2) The SA paskens that a chasan is obligated to fast during the week of sheva brachos.  The simcha of the individual is not doche the chiyuv tzom of the rabim.  Rav Shternbruch (vol 7 #246) asks why this should be true in our times when the chiyuv tzom is only a minhag and not still based on the takanas nevi'im.  I do not understand his question.  Minhag is binding like a neder.  If someone takes a neder not to eat matzah, for example, he is not allowed to do so even on leil ha'seder.  The minhag/neder to not eat on 17 Tamuz therefore of course is doche the obligation of sheva brachos.

R' Shternbruch distinguishes between the chiyuv simcha for a chasan and the chiyuv simcha on Yom Tov.  The latter demands eating meat and drinking wine; the former requires making the kallah happy.  It is not a chiyuv to eat per se, just a chiyuv to rejoice together.  Furthermore, that chiyuv is subject to mechila on the part of the kallah, and a mitzvah that is subject to mechila cannot be doche issurim (see Kesubos 40).