... I also kinda like the idea that Langtry is on the Grey Council. I could see him having a Snape-like arc in the end ...

I strongly suspect Langtry is more-or-less a "white hat" character; just with a bit of a dick-ish personality.

You don't have to be nice to be "fighting the good fight;" nor is being "nice" proof of being fundamentally "good" (after all, Lasciel's Shadow was almost-always "nice" to Harry; Marcone is routinely polite; etc).

And frankly, Langtry is both a brilliant politician & supremely well-connected; the idea that Ebenezar could institute a secret "Grey Council" with multiple WC-members right under Arthur's nose is unlikely at best.

He's also too bright to genuinely not be seeing the "Black Council" signs that Harry & Ebenezar were seeing, and almost-certainly knows that Ebenezer's exactly the sort of person who's going to ignore the "official position" of ignoring those signs, and take steps to counter the "Black Council."

I don't expect either one of them are exactly "thrilled" to be on yet another wizards' council together, but... "needs must, when the devil drives."

... I got the impression that the tall lean figure was Langtree.

To be fair, Langtry is another who'd be described that way.

Jim's inordinately fond of "tall, lean figures."  His own Mary-SueProtagonist is another!

Amongst a dozen grey-cloaked combat-wizards, Langtry could possibly have passed unnoticed by Harry.  But I suspect Langtry wasn't there -- the description of his battle-array was specific, and unusual; and his staff was also unusual in that it was a pure white wood, unmarked.  I suspect Harry would have noticed.

... It doesn't have to automatically be something that Harry chose that set off a whole avalanche of consequence ...

No; but it's WoJ that the Mirror mirror book would be based upon a choice of Harry's (and makes good sense for a Harry-Protagonist novel, after all!).

My own headcanon is that the choices of a Starborn often carry extra weight, even up to generating whole new  branches of the multiverse.

...
And why does it take six books to reveal it? 

Because Jim wants to keep the whole "Starborn" thing behind the curtain, do a gradual reveal.

I fully expect to find an upside/downside thing going on.  Maybe even that -- at the core of it -- being "Starborn" is an Outsider-created thing:  they force some power into the world, and it opens a doorway that they can slip through 40-50ish years later... but that original "force some power into the world" is what creates the Starborn.

I strongly suspect Langtree is on it but I don’t recall any confirmation that Langtree is a member.

Agreed.

Also:  I suspect one of the things that gives Ebenezer a fair degree of heartburn is keeping Harry & Arthur each unaware that the other one is also secretly a member of the Gray Council.
 

Langtree is on it, he was mentioned if I remember correctly in Changes when they finally showed up at CI.. Eb was also part of it, but off the top of my head I cannot remember who else showed up.  I wouldn't be shocked if we find out that Morgan was part of it and that Rashid is part of it..

Eb and Odin, I recall.  Lea (and/or Mab, by proxy) too.
I don't recall any clear signs of Langtry being there...?   (there is a puppy on my lap, I cannot go get a copy of Changes to re-read the scenes at Chichen Itza).

Didn’t Molly mention having significant financial resources as Winter Lady …

Molly does have significant financial resources.
I'm not _at all_ clear that she didn't earn those herself.  I rather suspect she did not inherit any "Winter Lady Wealth" per se.

She gets the asset of Arctis Minor (which sits in the Nevernever, and she cannot sell), and she gets the powers of the Winter Lady's Mantle.

But a smart girl like Molly can leverage these things to an incredible degree.  I expect she could have become a multi-millionaire within a month of Mab releasing her from whatever "basic training" she had imposed.
 

The one I'm most looking forward to is seeing what equivalent upgrades darkside Harry got that lets him take on the Winter Knight in a duel

IIRC there's a WoJ saying that mirror!Harry is closely-allied with Mavra.
Suggests to me that he likely leaned-in on the necromancy.

If I recall correctly the "point of divergence" is supposed to be some event towards the end of Grave Peril.

I'm pretty sure this is correct, yes; specifically, it's a choice Harry makes.

Exactly what choice Harry makes -- the critical divergence -- is, I think, an important issue.

Suppose he takes Bianca's deal -- having "cost" Bianca her playmate, he lets Bianca take the (already half-turned) Susan from him.  Immediate consequence -- Susan is a fully-turned Rampire, and Harry's self-image is "I traded-away the woman I loved, to be safe."  Note this choice is actually foreshadowed in the book:  Jim wrote how hopeless and defeated Harry was feeling.

He didn't expect to win that fight; it was a last-ditch moment of inspiration to summon all the shades of the Rampire victims to get revenge (and a startlingly powerful act of necromancy, given how untrained / unpracticed Harry was; he's clearly a natural with necromancy!).

I'm sure many of us can project all sorts of futures from that divergence.

On the other hand, let's take the choice at the very end:  after all the action's over, Harry tracks Susan to a beach.  They talk; she leaves.

Suppose he didn't let her, argued/persuaded/cajoled, "let's just try, for a bit, you can always leave if it's not working," and half-Ramp Susan stayed in Chicago.  But, without going down to Latin America, she likely never came to the attention of the Order of St Giles, never got their specialized half-Rampire training, etc.  No Maggie Jr, no Red-Court Takedown at Chichen Itza.  I bet the Reds won the war against the White Council; maybe (probably) a few WC wizards survived... they likely all hate Dresden's guts for starting the war.

Etc...  I think I've seen at least half a dozen different "critical choices" proposed, with more or less convincing arguments.