A Tish'a B'Av thought

The practices of Tish'a B'Av reflect a coincidence of three distinct themes on one day:

1) Day of mourning, akin to shiv'a. For example, not extending greetings (or responding to greetings before midday), not sitting on a chair before midday, not learning Torah, not wearing tallis or tefillin before midday. For that matter, one could also refer to the prohibitions against washing, anointing, the wearing of leather shoes, and engaging in marital relations. What is interesting about the first group of prohibitions listed is how it contains all of the prohibitions that are loosened at midday. If I recall, the prohibition of responding to greetings during shiv'a ends after three days. The prohibition of wearing tallis and tefillin probably has to be loosened at some point because we don't want the rabbinic law of Tish'a B'Av to prevent us from observing the Torah-mandated law of wearing tallis and tefillin. But why is there a difference between sitting on a chair and learning Torah? One of the baalei bayis in the shul at which I spent the fast asked me this question, and the best answer that I could venture was that not sitting on a chair is a more extreme form of mourning (which he interpreted as meaning that it was more difficult to keep while in the concluding hours of a fast), but this answer (nor his answer) sound so satisfying to me.

Tangent 1: Why is it problematic to wear a tallis on Tish'a B'Av? Presumably, the same law applies to someone during shiv'a. Regarding wearing tefillin, we derive the issur from Yechezkel, who is told that his wife will die but that he should still wear his tefillin ("Pe'eircha chavosh alecha"), but I don't recall why the tallis cannot be worn.

Subtangent 1: We also learn out the prohibition for a mourner to wear shoes from that parsha in Yechezkel, although the exact wording escapes me. But what is the source of the prohibitions of washing, anointing, and marital relations? See tangent 1 of part 2...

Tangent 2: If we're so concerned with a rabbinic law not preventing the observance of a Torah law (and note that invoking the Torah-mandated law of following rabbinic injuctions or the concept that the rabbis made their takanos with the strength of Torah-mandated laws do not answer this problem), what of not wearing tefillin on yom tov sheini shel galiyos? Even though originally it really was impossible to determine which day was Torah-mandated and which rabbinic, what of now, when we only keep a second day of yom tov because of the concept of "hizaharu b'minhag avoseichem b'yadeicheim", that we should continue to follow the diasporic practice of keeping two days of yom tov? It would not surprise me if yom tov sheini shel galiyos had some strength beyond that of a regular rabbinic law, but I can't figure out how this would work.

2) Day of not engaging in physical pleasure. Expressed by the five innuyim of Yom Kippur: Eating, washing, anointing, leather shoes, and marital relations. All of these are kept the entire day. Just like we do not want our physical needs to drag us down from our spiritual aspirations on Yom Kippur, nor do we want our physical needs to drag us away from our mourning on Tish'a B'Av.

Tangent 1: Maybe not engaging in physical pleasure is a law of mourning, as all of the innuyim of Yom Kippur also apply to a mourner. The exception is eating and drinking, but perhaps one can say that this would be too difficult to keep for a full week. The counter to this latter argument would be that at the very least one should keep it during the first day of mourning, which is Torah-mandated, if it is not possible to keep it during the rabbinically-mandated week.

Subtangent 1: If we say this, then it must also be true that sitting on a chair, learning Torah, extending greetings, and wearing a tallis do not constitute physical pleasure - and this likely is true (even sitting on a chair).

3) A festival. Expressed by not saying Tachanun. The source for this is the verse in Eicha "Kara alai mo'eid lishbor bachurai" - "He has declared a mo'eid against me to break my choice youths". Mo'eid usually literally means set time, but it is expounded here to mean a festival. Is it possible that we are celebrating the destruction of the Temple because of its preferability over the other option of the mass-extermination of the Jewish people due to our sins, much as Assaf initiated his chapter of Tehillim about the churban (79) with the introduction "Mizmor", which the gemara explains in the same manner which we have put forth here?

Tangent 1: Is it possible to explain other instances of the word mo'eid as referring to a festival, to lend additional strength to this d'rash? The first place that occurs to me is in the message regarding Yitzchak's birth in B'reishis 18, which fits because we say that he was born on Pesach.