Midwesterner your “points” are without merit:
“TZom Gedaliah was established by the same Chazal who established Tisha B’av in response to Churban Bais Hamikdash.”
That may very well be true, but it disproves the notion that we have no additional days of mourning after Tisha Baav was instituted
” Sefira is not an official azkara, only a miut in simcha. Anyone who has ever been in availus and learned the halachos should know the difference. And anyways those date back to tanaim, who were authorizeed to make such takanos.”
Sefira is a period of aveilus(During Nissan btw, at least for many) you can try to make artificial distinctions from today until tomorrow, and no it is not from chazal. Taanis Esther isnt from Chazal either, though granted it isnt really aveilus
” The crusades are b’davka mentioned in kinos on Tisha B’av (Mi Yitein Roshi Mayim, Sha’ali Serufa Baesh, and others.)”
20 Sivan was originaly for the crusades and in particular for the community of Blois that was burned
” And Tach V’tat was only imposed on the medina that suffered.”
So That would mean you have no problem with Survivors and their descendants commemorating Yom Hashoah