Coffee your follow-up was not mentioned in your original post but you did ask this previously. And I believe I responded then as well.
The time of the calamity is the time for us to pay attention enough to care about and internalize the loss. Are you truly suggesting that instead of Chazal making a day of mourning they should have made another purim to give mishloach manos?
Besides, it is a mistaken notion to think that the whole Galus is all about sinas chinam. It does say that that is why the Bais Hamikdash was destroyed, just like the first one was because of the three sins. However, that is not too say that it’s all that is necessary.
It’s obviously an important lesson of how serious Sinas Chinam is, and it’s a pretty good time to avoid it, but it’s not a day of friendship. You got all year for that.