I missed something. How do we know where the mirrorverse diverged?
Word of Jim:
https://wordof.jim-butcher.com/index.php/word-of-jim-woj-compilation/woj-on-future-df-works/Dresden as he would have been if he made one choice differently... the big decision at the end of Grave Peril
There's been a fair bit of discussion and debate about what "the big decision" is; even about what "at the end" of the book is... how FAR back from the end.
My own vote is from the very, very end of the book -- Harry lets Susan walk away. He could have argued harder, probably could have convinced her.
The fallout from that -- Susan never goes to Latin America, never gets recruited by St. Giles down there (maybe some North American branch reaches her?), so she never places any Little Maggie with the peasant family, the Reds never get their ammo for the Bloodline Curse (and Harry won't have boomerang'ed it on them), etc etc etc...
Mirror!Harry then has a local half-Ramp ally in Susan, like he has a Whamp ally in Thomas; tactically, Harry is MUCH more dangerous; strategically, it's a disaster: you can just IMAGINE how much the White Council would love
THAT, and the odds of escaping their notice drop exponentially (since everyone knew that Harry & Susan were "a thing"). They probably figure out he's gone over to the Vamp side of things, and is one of the Ramp/Whamp cooperation channels.
Having Harry!Prime meeting Mirror!Susan will F*** with his mind in all sorts of fun ways, I'm sure Jim will love the chance to torment Harry that way.
I also argue that this is "the big decision" because it was
THE big decision -- the one about Love.
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My second choice choice-point is at the beginning of the battle with Bianca -- she threatens him with open war, points out she already has Susan half-turned and the other half is inevitable.
And Mirror!Harry caves.
He takes her bargain, walks out with Justine, yielding the field to Bianca. In this case, Mirror!Bianca is alive, Mirror!Susan is full-Ramp (with an extra hate-on for Harry, for not saving her).
That decision would ALSO have been about Love; and Mirror!Harry will have such an abiding self-loathing after that, I can imagine it beginning his slide down to the dark side.