I think the reason for the book being so many chapters is that he wrote 1/3 the book and then ended up having to take two year break on writing....when working on a project like a book you never stop thinking about what you are writing even if you cant write. I bet Jim Butcher came up with another plot line to weave into the story and ended up rewriting a great deal of the chapters he all ready had written. this new plot line ended up also adding another 12 chapters to the book....this is my guess and if I ever meet him at Q and A I am going to ask him. if I am right I think the plot line was created when he wrote the short story Zoo day.
I've seen it happen to other writers in the past. It's not quite boredom with a current direction so much as greater excitement about a new addition. I would not be surprised in the least to find out that Jim got some new ideas for ongoing plotlines and resolutions to existing ones while poking at Peace Talks in his head, considering the sheer size of the cast and the opportunity for foreshadowing and straight-up exposition. Good God, I'm getting sweaty just thinking about a thirty-page Ebenezer Infodump about the Life, Universe, and Everything.
I'm also willing to bet that Jim spent a lot of time thinking about the Cinder Spires, even though he was still slated to finish Peace Talks before continuing that series. I'm on my annual Butcher Binge right now, listening to all of his books through Audible this time, and I'm reminded about how much fun the Aeronaut's Windlass is and the opportunities for worldbuilding, political intrigue, and cat-on-
everything violence he'll have going forward. I'm sure he's got some new plans for that series, too.
By and by, is his next work after Peace Talks (and, I suppose, the Maggie and her Maggical Pals series debut) confirmed to be a Cinder Spires entry, or is he jumping straight to Mirror, Mirror? I'd heard the former, but I saw some speculation that he'd be sticking with Dresden to try to get back on schedule.