I was watching his Twitch stream last week. He was playing through Portal for the second time and he starts talking about narrative structure and how a game like Portal, with its puzzles and plot reveals, loses quite a bit after the first play through.I'm pretty sure that he's hit a block - there's no way he can take the series from where it ended in #2 up to the "present day" framing device and wrap the whole thing up in a single book, but for some reason he refuses to bite the bullet and expand the length of the series like every other fantasy author does.
It's a shame, because imo the series doesn't have much re-read value. I devoured it the first time through (admittedly, I felt #2 was a drop from #1 even then), but found it just above average on the second read of the first book, and felt no motivation to move onto the second book again. I'd just like some closure on the story, but doesn't seem to be coming any time soon.
He then immediately starts explaining that his books are designed so that you get more out of them with multiple readings.
Not saying I agree, just that your post reminded me of what he had said.