Either Susan's or Lea's love for Harry.

To harm the white court, it has to be romantic love between equals (i.e. parent-child love doesn't harm them) and it has to be reciprocated. Harry hella definitely isn't in love with Lea. Occasional lust maybe, but mostly he's afraid of her.

Who says he's in a psyche ward because nobody believes him?
Maybe he's in a psyche ward because he actually has some unrelated mental issue.

A werewolf who actually is unstable could be fairly terrifying.

What was the catalyst for this? People don't wind up super religious or institutionalised or in deep denial without some sort of catalyst.

Side effect of "Marcie" the deep-cover supervillain messing with their heads?

He said, half-jokingly. Half.

Is this all that unsolved? In hindsight it sounds Outsidery as all hell, even if the specifics aren't confirmed.

I can think of three.  Mai, Liberty, and Luccio.  I don't count Molly any longer.

Luccio cannot be Kumori, as explained earlier.

Mai or Liberty can't be so definitively ruled out, but it would have been logistically difficult to split their time between running around in Chicago while the Senior Council were engaged in battle in Dead Beat. Plus neither of them are particularly young, and Kumori said during their one-on-one conversation that she and Cowl had been taking turns trailing Harry, presumably including climbing all those stairs to go talk to "Sheila".

Earlier, when Harry/Thomas are trying to get away from the Scarecrow, he causes it to slip on ice.   On the face of it, this is a little implausible.  After all, we've seen (Cold Days) that even the Winter Knight has no problem running on ice, and this is a power being from deepest Winter.

Thomas also did way the hell less damage cutting the scarecrow with a steel sword than one would ordinarily expect from a usual Fae.

I would suggest American too.  I don't recall Harry mentioning an accent.  Unless magical shielding was going on, I'd doubt it was Luccio.

They were muffling their voices. But Luccio is one of the few female wizard we're sure cannot possibly be Kumori, on account of how she was busy being saved from almost bleeding to death by Butters while Harry and Carlos confronted Cowl and Kumori.

Harry might not be curious about Kumori because he assumed the apprentice's shield wouldn't have been strong enough to survive the blast. But it does stretch suspension of disbelief that after confronting Cowl again at the Raith Deeps in WN, the realization that he'd lived seems to just have been filed and forgotten. Another villain left out there who probably still has the Darkhallow committed to memory is a rather serious Chekov's Gun to just ignore.

It's possible Harry has put an unsuccessful effort into investigating some of these things - Jim puts it that the case files are basically his worst weekend out of the year - but failing to run that warning up the chain to Morgan and Luccio at the end of the book seems odd.