I love all the Mirror episodes of the Trek series, but curiously I am not interested in the DF: MM book at all (other than because it would move the story towards the BAT). I just don't care for an alternate Harry. I just want to see more Harry (Harry prime 
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My guess is while the Mirror Mirror novel will most likely read as almost a stand alone story that is seemingly separate from the rest of the Dresden Files we know after Grave Peril; at or near the end of that novel Harry (prime) will learn several things he couldn't have learned in his own universe. As you said, these bits of knowledge will help move the overall story arc towards the BAT.
This is probably four years from when Mirror Mirror comes out (God, that is depressing.), but I hope it is a great stand alone story, because I would mostly be reading it just to find out what Harry prime is going to learn. One TV show that Jim and many fans are familiar with is Babylon 5. They used to have episodes; particularly in the first two seasons, that were clearly meant to stand alone from the overall story arc. Some of those episodes were very good, some were really weak, particularly in the first season of the show. However, even in some of the weaker episodes there would usually be some tidbit of information casually dropped into that episode that would become significant in the larger arc of the show. Whenever it comes out, I hope Mirror Mirror is a real page turner, because I don't want MM to be like one of those second rate B5 episodes that are only important because of the odd scraps of information that gave us clues to what was coming next.
P.S. Also, whatever Harry learns had better be worth the wait.
Methinks you forgot an operation. It was chapter 36 on 12/7 and 38 on 12/17. That's two chapters in ten days, averaging a chapter every five days. If there's 14 to go, that's 70 days, rather than 140. Maybe mid--to-late-March, assuming he takes some holiday time off from writing?
I don't remember where I heard or read this, but I remember Jim stating that he can average writing about 4,000 words a day when everything is running smoothly. (Or was it 3,400 words a day? - Just ignore that and pretend it's 4,000.) The last four novels had these many words in it: Changes: 149,280 words, Ghost Story: 162,899 words, Cold Days: 175,685 words and Skin Game: 151,922 words. That averages to about 159,947 words. So Jim has finished 38 chapters, and I believe someone has said most of the later books have averaged 52 chapters. Assuming most chapters average out to same length, that would mean Jim has written about 116,884 words so far. (159,947 words / 52 expected chapters x 38 written ones) This leaves just over 43,000 words left for Jim to write. (159,947 - 116,884 = 43,063)
In theory, at 4,000 words a day Jim could be done in less than two weeks, but I doubt that will be the case. (Of course, we have to add at least a week for Christmas and New Years.) As Jim approaches the final chapters, I think it's to be expected that it takes longer for him to work out all the details, so the climax of the story isn't just exciting, it also makes sense and fits neatly with what has come before it. So my guess is if Jim is on a roll now, maybe he will finish in about 4 to 6 weeks. I hope those updates on his website keep coming. It's nice to have something to keep track of.
(I know Jim needs to take a break from Harry when he finishes a DF novel, but I really wish he could just take three months off, maybe travel to Australia and New Zealand (or wherever). It's Summer there now. Shelve the next Cinder Spires novel for a while. Then come home and jump into Mirror Mirror. IMO the Aeronaut's Windlass was OK. It had some fun parts and there was at least one character that I think might become really interesting down the road, but I can't say I'm dying to read what happens next.)
