WOW, man...  Lotta content here!
Very meaty, very tasty!

I've been thinking about Twelve Months and I think it will be a different type of story than anything we've read so far in any of the previous Dresden Files case books
...
So, that has me asking what the through line will be; the main story, that will flow through the entire novel ...

I think you're 100% correct, it's going to be a very different book.

As I recall, Jim has stated 2 broad elements we'll see in this book:

 - Harry working broadly in Chicago, as a more-significant figure there:  The "Wizard of Chicago."

The guy lives in a freaking Castle, now!  Scores, maybe HUNDREDS of people sheltered there, during the Battle; many more saw him in action.  He raised a Banner, and almost a thousand mortals followed him to war.  And those people in turn each know multiple people.  The number of first-hand accounts (and still more people, who can vouch for those first-hand witnesses) is into the thousands, maybe scores of thousands.  That's a small-to-medium town's worth of people.  Sure, it isn't enough to be a sea-change for big-city Chicago overnight (that would have been a win for Ethniu)... but it's enough for a perceptible change.  At a visceral level, much of Chicago now believes in magic.

So, Harry's not nearly as low-profile as he used to be; and the Castle is a big, tangible resource:  expect Larry Fowler's lawsuits to resume!         Expect more lawyer-time, tax bills, and other "interface with mundanes" elements.  I expect he'll want to follow up with his Bannermen, see how they're doing, their families, etc.

But also, I expect Harry to be reaching out to the local Paranet, the Better Future Society, and other low/mid level practitioners and clued-in folk, forging those alliances, too.

Harry's SOP was "Big Damn Hero (with some friends);" he had the low-levels hunker down.  Well, the Battle of Chicago showed that there is no hunkering down, not in the new magical world:  Pure mortals had to show up & fight, too.

So, "Harry, living his new life in the new Chicago" will be a theme.

 - Jim, who has experienced a lot of pain & trauma of late, "writing what he knows:"  showing depressed-Harry, traumatized-Harry, grieving-Harry.

Grieving for Murphy, of course:  that's the big one.  But in the Christmas Eve short, we know that Harry has shouldered a big chunk of grief for Chicago-at-large, all the deaths & injuries &c from the Battle (yeah, that's not rational... but feelings often aren't; Harry spent over a decade trying to protect mortals from the Supernatural World, and the Battle of Chicago likely felt to him (at some level) like a massive personal failure.  Plus, we know "shouldering extra guilt" is one of Harry's specialties!).  And you know there are dead parents among his Bannermen, leaving orphans and spouses, and Harry feels very directly responsible for leading them to their deaths.  I think that will be an element, maybe even a substantial theme.

Lots of meditation, and who knows where that might lead Harry.  I suspect he'll discover some about his links to Demonreach, possibly other stuff.

I suspect we'll be seeing Agent Tilly, again.  Likely he's been recruited by the Library of Congress' Special Collections, tho maybe the FBI has their own Black Cats / SI now.  But the Fed has *got* to respond to this... even if only to report it as a terrorist EMP device & hallucinatory nerve agent (but that's only the Official Story, and this coverup can't be hidden by a small cadre like the Chicago PD did with BC/SI:  the higher-ups will have proof now, so they will be launching plans & investigations &c; and that will involve Harry).
 
Lara:  once Lara gets over her surprise, she's going to realize that obviously Mab Has An Agenda, and it's nothing as simple as a marriage (not even a not-at-all-simple "Marriage of State" between Winter & White courts).  So she wants to know what that agenda is.

I don't think she's going to be too terribly devoted to trying to seduce Harry:  WoJ says the WK-mantle and Whampires' respective sex-mojo's combine in more-than-additive ways, and I'm certain the White Court has decent records and likely there's some Raith family-lore, too... Whamp's & Winterfae have been co-Supernatural'ing for thousands of years, after all!

At the same time, I think Lara's got a bit of a crush on Harry:  her control keeps "slipping" around him.  She just cannot seem to help but poke that particular tiger, but we can be fairly sure she's actually got a lot of self-control (in general).  So I expect ongoing, low-grade "ooops" events.

And, on the gripping hand, Harry just sorcerously-imprisoned her only brother after "tricking" Lara into thinking Thomas would be enjoying relative liberty (and she could visit him) on Demonreach Island, and she is furious about that.  I wonder if that resentment (and desire to get inside Demonreach to extract her brother) will be the leverage Nemesis needs to take Lara...?

Which brings us 'round to Mab's agenda again (or should I say agendas:  I expect there are several!).  One is further training her Knight to conduct socio-political battles, as well as physical ones:  how to extract himself from the engagement without provoking a White-Court / Winter war.  I guess Mab also has got "Major Crush On Harry" Molly being intimately involved in planning Lara&Harry dates (?), which tells us that Mab has a substantive agenda here for Molly, too.

But my WAG as to the most-overt of Mab's agendas is that she has assigned Harry to hunt Outsiders among the Whampires:

• Papa Raith showed up with major Outsider-fueled magic protection -- the Blackstaff was repeatedly unable to nail him.
• Vito showed up with a major Outsider-fueled psychic assault, taking down multiple high-level opponents at once.
• Justine showed up with a Nemfection, while working very-closely with Lara Raith

“Once is happenstance. Twice is coincidence. Three times is enemy action.”  I think Mab thinks there's a problem in the White Court (I think she's right).

Also:  Raith Père is supposed to have an amazing library concerning the whole Starborn Cycle (he was trying to educate himself into becoming a power-player for this cycle).  I think Mab wants Harry to have access to that library (I suspect the collection is more than "just books," and includes a Nemvector).
 
The Blackstaff:  I doubt Ebenezer will just let things go.  He still wants to save Harry from himself; he still wants to get to know little Maggie better; he still has his anti-Whamp attitudes (which, to be fair, are largely justified).  So I'm expecting family drama there (with the added complication that the White Council's Black Ops guy is Grandpa, and the WC's highly-suspicious Enfant Terrible is Dad... and then there's Maggie's "little" sister Bonea ... ).
 
The Marriage:  Twelve Months is the time leading up to the wedding.  Mab granted that when it was pointed out as inappropriate for the groom not to have a reasonable time to grieve for his fallen love.  So the title of the book is right there.  At the end of the book, there will be a wedding.

My WAG is that Harry -- after all his & Molly's & Lara's twisting and turning to try escaping from the marriage -- is getting into his Tux (or whatever the Winter Knight wedding-regalia is) and about to marry Lara.  He pauses for one last look-over in a mirror to make sure everything is OK, and his reflected image reaches out from the mirror & grabs him.

And that's the last scene of Twelve Months.
Then we just wait for Mirror, Mirror

And at the end of Mirror Mirror, he comes back to his own world having stood-up Lara at the altar (which for a very-proud Succubus Queen is... well, the phrase "mortal insult" comes to mind) AND simultaneously dishonored Mab's sworn word (which is... well, "suicidal" comes to mind).

Then we wait for next-book.

yoooi seriously, stip repeating exactly whf I just said likewise just didn't day it... 

Except she wasn't just "repeating" it.

For one thing, you used a specific jargon term, "aspect" (his aspect of happy light fire) which (depending on who is reading, and how much they know of the "extended canon") might be taken specifically as a reference to game-mechanics from the Dresden Files Roleplaying Game, where "aspects" are a Really Big Deal, and only change at "milestone" events.

For another, she was explicitly disagreeing with you (more to do with his own emotional state... than any floodgates he opened).

Jim said it during an interview. It's on YouTube the youtuber is Jr Carrel, the video is called Exclusive interview with Jim butcher: unveiling the magic behind the dresden files. It's at the 1:22:34 mark

Thank you!

I assumed Toot was talking about Native Americans.

Possible; I don't think we have enough info.  It's also possible he means an "old country" reference -- the pixies are a very Euro/Brit sort of fae (not New-World) so he might have been talking about the Celts before the Anglo-Saxons, or the Picts before them, and so on back in time.

He also may have been making a more mythological reference -- maybe to the Tuatha de Danann or the like?

Or, on the Native American theme, to some of the mythical early-human / pre-human peoples of the new world.

Again:  I don't think we have enough info.  It'd make an interesting WoJ question at a signing or AMA event!

And, to return to the OP:

... throw away context or relevant?

Yet again, I chant the refrain:  I don't think we have enough info.
 

...   Is the black staff her walking stick?

That is the general presumption hereabouts, yes.

... Rufus...

errrr... Rudolph?

I'm pretty sure Jim has stated that Mister is just a cat (the Malk theory has come up before, including (I think) during some sort of Q&A or AMA).

Obviously, Jim could have been lying... or can just change his mind.
 

I wasn't thinking of it as a teaching role but more of police function or even a position similar to Mab's but covering magic. Harry would get a limited Intellectus to warn him of major magical threats and events and have the newly formed Wardens as enforcers and the Paranet as the administrative and education arms of his new council/court.

So far as we know, all mantles are constructs, made by an entity for a purpose.
Who could (and would!) create such a mantle?

I suppose the Archive has the knowledge to create it; and there's enough power on the Island to do the implementation... but, why would she?  Her Oblivion-War duties are big on secrecy, and adding a new & independent globe-trotting Mantle to the mix looks like a huge gaping OpSec hole...

The Gatekeeper sent Harry that cryptic "black magic in Chicago" message; so he seems to already be capable of doing "black magic scans" & the like (though  that could have just been smoke & mirrors, with Mab tipping off the Gatekeeper to feed Harry the bait, so she could reel him in...) .
 
Also, Harry "I've got Authority Issues" Dresden would himself be a really ironic target for becoming the ultimate Authority Figure on the issue of Magic!  (this is not, I should note, any argument that Jim wouldn't inflict such a thing upon Harry!)