I was thinking about Edom as a parallel dimension, and how Thule had a crisis point where everything was changed. The Battle of the Burren.
With Thule, because it's so similar to the TSC reality, it's clear what the crisis point is, but with Edom there must be a million smaller things and some bigger things that happened differently.
The change happened in Jonathan Shadowhunter's lifetime. And what hit me was that Alec said no Silent Brothers or Iron Sisters were ever created... but the first Iron Sister and Silent Brother were Jonathan's best friend and sister... so in Edom, what happened to them?
Did they even live to become Shadowhunters, or were they gone before that?
Jonathan Shadowhunter in Edom was a very different man. He didn't ally with Downworlders, which the Nephilim should have been doing from the start (with Elphas the Unsteady for instance), so what happened? Obviously it could be anything from during their lifetime, but I at least get the impression it was something to do with a decision Jonathan Shadowhunter made which changed their entire fates. Something that defined him as a leader, a difficult choice between a hazy right and wrong. But what made him choose wrongly? David was his friend and source of wisdom. Did David die? Or were they never friends at all? Was his sister Abigail killed and he was responding to the loss? It could have been anything.
I'm just going on what's loosely described in the stories and particularly in the Codex, so this is all vague speculation, but it makes me wonder.
The biggest crisis point in his lifetime is the moment Raziel appears when they're about to die and asks him what he wants. What if he didn't answer the same? What if his priority had not been to save Abigail and David? What if he had said something else?
That would have changed the course of their history drastically. It would have changed the man Jonathan became, it would have eliminated the effect David and Abigail had on Shadowhunter society respectively, and it could easily have lead to a self-destructive Nephilim like the ones in Edom.
In the Codex, the writers ask you to forgive Jonathan for not asking for something greater than the survival of his friends, as if this was somehow a lesser concern. And for the main body of the Clave, it's always been that way - they are seen as cold and acting superior and often cruel. It's the Iron Sisters, the Silent Brothers, and a handful of open-hearted young Shadowhunters that usually balance them out. But what do you get if you take those things out of the equation?
The friends that Jonathan saved (supposedly unwisely) become key to the survival of the Shadowhunters, and the rest of the world, through the things they did. What's more, that moment where he chooses to save them is a direct parallel to when Clary is offered a favour from Raziel and she chooses Jace's life over anything else.
Perhaps Edom's destruction represents what happens when the Nephilim choose their lofty moral high-ground and solitary superiority over love and unity and friendship. Perhaps it is the extreme of what they would become, if they'd been doing that FROM THE VERY BEGINNING.
And perhaps, to contrast to Clary's guilt over choosing Jace (since it was the only reason Sebastian could be brought back to life, causing the war, the very reason they're stuck in Edom at all), it represents the importance of that choice. That putting the safety of those she loves over anything else is not selfish, but it gives her the humanity and love and strength to make the right choices and win the war in the end.
It's the direct antithesis for what Jace told her in the very first book 'to love is to destroy' - proving that love is, in actual fact, the only reason that anything survived at all.