Okay so while out of the given theories here I find the 'unspeakable tragedy' most likely, it just feels like it's too easy. After all, our main evidence comes from a conversation between Nazh and Kelsier. What every happened to "there's always another secret"? I find it plausible that something else may have happened, or some other force is behind this.
Also I remember a quote from the story that could easily be glanced by. Silence was speaking to one of her daughters and she said "Justice died in the homeland". While I could easily be overthinking this and it could just be nothing, what if she wasn't referring to the judicial concept of justice? What if, instead, she was referring to another shard? The shard of justice. Could it be possible that after Odium's fight with ambition another shard made its way to threnody, perhaps while Odium was distracted? Or maybe, the problem is that Odium wasn't distracted. If Odium noticed perhaps he came and destroyed justice and then that's what caused the "Evil" in the homeland.
Finally, Brandon has said that Hoid doesn't appear in this story, however Peter Ahlstrom has suggested that he does appear. Could Hoid be Earnest? After all we know that Hoid generally appears before bad things happen, and Earnest appears at the beginning of the story and then once more at the very end.
Anyway let me know your thoughts on my probably half-baked theories. I'm certainly not the most knowledgeable on the cosmere so let me know any holes in my theories so that I could perhaps adjust them or even throw them away altogether. Thanks!