Note - I subsequently learned that Congregation Aish Kodesh of Rabbi Weinberger had their own kumzitz on Tisha Be'Av night, although it was not advertised as the Lawrence congregation one was. A video of it, with Eitan Katz, can be seen here.
Now the three weeks period, between the observances of יז תמוז and ט אב, can be a challenging time for many people. Especially in the COVID-19 era. For Chasidim and Neo-Chasidim, it can be even more so, since they put so much stress on שמחה.
It seems that for years Tisha Be'Av 'kumzitz'en of sorts have taken place on/around Tisha Be'Av in Eretz Yisroel, especially around the כותל המערבי. However, it seems that they were usually done by groups like NCSY, and also more toward the end of Tisha Be'Av, rather than in the beginning of it. Now by longstanding widespread custom, toward the the end of Tisha Be'Av, there is a shift in the mood, for example, after חצות היום most people sit regularly rather than down low, so such things are not so surprising toward the end of the day. However, to see it advertised in a Shul setting for the beginning of Tisha Be'Av, and in חוץ לארץ too, was surprising.
Another such innovation, coming from the same or similar circles it seems, is to sing together אני מאמין באמונה שלמה בביאת המשיח on Tisha Be'Av night after davening. I have seen that practiced at Cong. Aish Kodesh in Woodmere of the aforementioned Rabbi Weinberger.
Now we can assume that people have good intentions with these innovations. But nevertheless, the question can be raised, are they a good idea, and consistent with our מסורה?
Personally, I don't think so, I think they are definitely a departure.
An excellent article related to this was written a few years ago by Prof. Chaim Saiman.
If people feel an overwhelming need to do such things, esp. in a congregation of youngsters, or a kiruv group, perhaps there is room for leniency for some singing under rabbinic guidance. But does it need to be labeled as a kumzitz and formalized? To institutionalize them, especially at the very outset of the day, seems problematic.
Additionally, the idea of singing together of אני מאמין באמונה שלמה בביאת המשיח on Tisha Be'Av night after davening seems inconsistent with the theme and mood then, which seems to be describing and bemoaning the terrible, massive חורבן and calling for תשובה, repentance, to remedy the great breach that developed between הקב"ה and כלל ישראל. But if people start singing then that they believe in the coming of משיח no matter what, even if he tarries, that seems to dilute and take the focus and urgency away from תשובה as the answer to חורבן and גלות, because it is implied that משיח is coming anyway.
Those are some thoughts that came to me when contemplating these matters. תורה היא וללמוד אנו צריכים.
May we be זוכה to תשובה שלמה and גאולה שלמה, בקרוב.