Tish'a B'Av

On the night of Tish'a B'Av, following the shiur that I described in my previous post, I got into a long discussion with my future roommate Zev on some of the themes of the mo'eid, specifically based on a point that I threw in at the end of my shiur that the purpose of reviewing the history of the destruction that occurred is to aid us in correcting the causes that led to the destruction, as per the famous statement that any generation in which the Temple is not rebuilt is itself deserving of having had it destroyed in their own days. Zev summed up some of the ideas we discussed in a post on his blog. As follows is my response, which I am also linking to in his comments section.
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It would appear that the midrash which states that every generation in which the Beis HaMikdash is not rebuilt is considered to have destroyed it is predicated on the assumption that the Beis HaMikdash can only be rebuilt through Divine means - coming down from the sky, as it were, whether this is to be understood literally or metaphorically. As a support to the metaphorical sense of this opinion, I point to the attempts to rebuild the Beis haMikdash during the centuries immediately following the 2nd Destruction, most notably in the year 363, during the reign of Julian. In other words, even if the 3rd Temple will ultimately be rebuilt by human hands, until it is the Divine Will for it to be built, such shall not succeed: "For even if you had smitten the whole army of the Chaldeans that fight against you, and there only remained the wounded men amongst them, they would still rise up, every man in his tent, and burn this city with fire" (Jeremiah 37:10) This being the case, though, our belief is that that there will, indeed, be a third Beis HaMikdash, and it may very well be built by placing brick on top of brick.

When we discussed this topic on the night of Tish'a B'Av, I suggested a broad definition of "Ein mazal l'Yisroel" that could encompass a potential immunity of Israel to even the natural entropic driving forces that lead to all states' decompositions. In an essay by RAL entitled "Centrist Orthodoxy: A Cheshbon Ha-Nefesh", he makes a similar point. The laws of nature and probability that dictate the rise, fall, and ultimate destruction of nations can be suspended by HaShem when circumstances call for it. Such an open manifestation of "Ein mazal l'Yisroel" that we are discussing would only differ from our historical experiences until this point quantitatively, as it seems clear that on the qualitative level, Israel's continued existence for over 3000 years, both as a continuous physical entity and as a continuous ideological entity, is a phenomenon that has not been imitated by any nation in any region of the world that has exhibited political disunity comparable to that of the Middle East.

I agree that, looking at history until now, there have never been periods longer than mere instants during which the Jewish people has not been deserving of destruction on some level. This being the case, it may seem overly optimistic to hope, even leaving aside yeridas ha-doros and similar concepts, that we can ever truly be deserving of a more open manifestation of "ein mazal l'Yisroel". However, deservingness is not the only criterion which dictates the historical phenomena that befall the Jewish people. The idea is frequently expressed in NaCh that HaShem is not only driven to act for the sake of the deservingness of K'lal Yisro'el, but is also driven to act for His own sake. This being the case, the level at which we will no longer be metaphorically guilty of having caused the destuction may not be literal and absolute, but can also be a function of this second criterion, namely, God's honor and stature. This second criterion itself need not be constant, but can also be a function of different variables in the world, including, but by no means limited, to time. Striving to correct the problems that led to the destruction, therefore, is primarily of importance in the directional sense, in that even if we do not (and cannot) ever reach the point where we have fully corrected these problems, simply proceeding in this direction may be all that is necessary to make us worthy of the rebuilding of the Beis HaMikdash as a function of two variables.

I also agree with your characterization of Tish'a B'Av as a day governed by fate and crushing hopelessness, and thus one that is distinctly unsuitable for thoughts of repentance (at least until midday, when the day seems to do an about face and transform almost entirely into a regular fast day, on which one of the primary theme is repentance). Nonetheless, a significant theme of the day is tzidduk ha-din, accepting the harsh judgements upon us as fair, which is strongly bound to an acknowledgement of our sins that led to the catastrophes of this day - and this, itself, is the first step to repentance, although we cannot proceed any further along this path in our state of devastation on that day.

(As a side point, it recently was made known to me that my own shul on Long Island, until a few years ago, was probably one of the only shuls that showed part of the Chafetz Chaim Heritage Foundation video at night - there were good reasons why the rav, ZTzL, chose to schedule things in this manner, but I always found it a little bit disorienting, for the reasons that you said).

On a concluding note, even the days of the Mashiach do not promise eternal stability and equilibrium. I recall seeing a source that says that the Messianic era will be limited to a single millenium, following which the world will cease to exist. While there is much to delve into regarding the meaning of this statement, the point is that even the coming of the Mashiach is not the final answer to all of the world's problems (especially in light of the famous statement of Shmuel, quoted by the Rambam, that there will be no difference between this world and the Messianic world, save the single difference of Jewish political independence). Hence, even during the reign of the Mashiach, the forces of entropy will continue their unrelenting labor, bringing in their wake poverty, sorrow, sickness, death, and destruction, although kinetic constraints may still prevent these from coming to pass. In this sense, truly the only eternal stability is in the Gan MiKedem, in which the forces of death and decay are neutralized entirely.