This sounds like it might be a linked story /novella collection in which case Christmas Eve is already written to slot in as the mid point. This may be quicker to write as it is episodic rather than a chronologically tight narrative of the usual files and would allow for Jim to write chunks between other projects during the lockdown. We know BG took place on the 21 June, Christmas Eve 24 December, so I presume the latter is the turning point where Harry starts to properly heal. Very much a different book to that we know. Jim stretched himself breaking format with Zoo Day, he may be trying the same here and if Twelve Months is half as good as that story then we are in for a treat. I also wonder if this period is going to be when the Maggie and Mouse stories are going to be set, in parallel to the events of Twelve Months.
Mab had expressed in BG that she expected Harry to be seen with Lara at a number of events, doubtless they will feature. I would expect one to be Harry studying with River Shoulders, perhaps return of Gen? Another dealing with Bradley and Rudy. Perhaps one dealing with the n- ?. One or more dealing with the White Council trying to involve themselves in Chicago and/or the Paranet, there are plenty of story threads which can picked up, and allowed to develop over time so we see in the background Harry setting up Camelot, coming to terms with the deaths in BG, the evolving relationships with his children, the rebuilding of Chicago, the hunt for Justine and the recovery of Thomas.
As for Mirror, Mirror, just as Harry gets his life straightened out, his evil twin tries to steal it. Works for me as a set up.