Do we know that for sure, or assume that? ...
We don't
exactly know how far Mirror!Harry has stepped over that line. WE do know he's a much darker character than Prime!Harry.
WoJ has stated he's allied with Mavra, and has taken (when cornered) to summoning Alt!Harry's to die in his place; that looks like
pretty far over the line, to me.
... When you consider the original Trek story which supposedly this is the inspiration for Jim's story, we see what we think is "evil Jim Kirk" verses the good Kirk of our world.. However taken in the context of his universe, "evil Kirk" really isn't at all ...
I'm not much for moral relativism.
Sure,
some things are relative. But some
other things are outright evil, and not relative at all. Mirror!Kirk is evil.
Now, "it's not his fault" & all that -- he's evil because he was raised in that setting, where kindness was seen as weakness (& exploited as such). But being a
victim of evil doesn't mean he didn't
become evil as a nearly-inevitable result of being informed by that setting. It's exactly similar to how child-abuse is always evil, even when we learn that the abuser turned out that way because they themselves were abused as a child.
... because everyone with the exception of the Spock of that parallel universe is like that ...
I always suspected that Spock mirrored most Vulcans: being
ruthlessly logical is the logical Vulcan consequence of being embedded in (literally) an Evil Empire. But the Mirror!Vulcan history was presumably similar to the Prime!Vulcan history (i.e. driven by tradition & logic) up until Vulcan/Terran "first contact," and they adapted (in their logical way) to ruthlessly match the the circumstances they found themselves in.
... In the "evil Harry's" world, Murphy may be a corrupt cop with no morals at all and Molly might be entering a convent to become a nun! Then what? ...
Remember, though: there was no alt!universe or branching up until Harry's "choice" at the end of Grave Peril. There was only a single timeline up until that divergence. The rest of the universe isn't just up for grabs, with random alterations; the changes rippled outward, as alternative consequences resulting from Harry's alternative choice.
I don't think we have any reason to think Murphy would have become corrupt / immoral. Defeated, maybe; hopeless, and "just getting by," rather than crusading for what's right. What is Murphy (and SI) like, without having been able to rely upon Dresden? Less clued-in, less-successful; but what else?
This, I think, is going to be the core of Mirror,Mirror: exploring the consequences of Harry's choice. Even more than the ST:TOS episode
Mirror,Mirror, I think Jim's going to write an alt for Harry's own life: this is going to be Dresden's
It's a Wonderful Life (1946 with Jimmy Stewart). But also, of course, more of Jim's primary job: tormenting Harry.
You ask after Molly, and I think it's pertinent. In fact, I think we'll be revisiting most/all of Harry!Prime's "Scooby Gang" (Michael & Thomas & Butters & Bob & the Alphas &c), and the followups/consequences of his prior Casefiles & how Mirror!Harry handled them differently from how Prime!Harry did. We may even see the return of Little Chicago!

I'm unclear about the particulars of any of these, of course... it depends how Jim writes the alt!timeline. Speculating
specifically about Molly (and Michael), though: Harry has become a serial murderer & Blampire-ally; I don't think Michael is his friend, or ally, or trusts him in any way. So, Molly hasn't idolized him: she hasn't seen him as a mysterious badass, fighting monsters (alongside the father she adored). Presumably, Mirror!Molly began developing powers just as Prime!Molly did; but without Harry's example perhaps she turned more towards her parents & they helped her tamp them down? But I suspect empowered Molly is too narratively-useful to Jim, so we'll see
some sort of Magical Molly (unless we learn that she got executed by the Wardens -- very much a torment-for-Harry element!).
I suspect Prime!Molly was guided by Lea or Mab toward her Black Magic path, specifically to use Molly to entangle Dresden; but without the Michael/Harry alliance that entanglement wouldn't happen; so no Winter shoving Molly down the left-hand-path.
In fact... I suspect Harry's "choice" in
GP was basically one made from being scared, hopeless. From giving up, instead of fighting-against-all-odds. I'm not at all sure Mirror!Mab would even want this weaker Mirror!Harry as WK.