Well GRRM explained it as follows.
The book was still incomplete and it was 80 (book) pages longer than aSoS. He thought it would take an extra 100 manuscript pages to finish it fully. His publishers were really anxious to get it published this fall so he was left with 3 choices.
1) Find a quick way to wrap up the remaining chapters and try to sweat the stuff he has already done to reduce the size of the book. Even though it was unfinished, it was already too big for them.
2) Cut the book in half by timeline, publishing the first half this fall.
3) Cut the book in half by POV.
The first option was a bad one in my opinion. You don't want the author to cut chapters, or characters because of the publishers. GRRM didn't like that either.
The second one has advantages in that it updates us with all the POVs but OTOH it leaves the stories all unfinished. Everything just suddenly ends.
Option 3 means a tighter book. We get 7 or 8 POVs like aGoT, rather than 15. We don't get lost by the huge number of plotlines.
GRRM said Dany and the Wall is excluded. That removes Dany and probably Tyrion plus the Wall which presumably means Jon and Davos. Probably Sam too although he may be going south. This leaves the Dorne chapters, Jaime, [Note: Spoiler POV redacted], Sansa in aFfC. Arya is hard to say. Similarly the Iron Islands. The latter seems more like the North to me. But I haven't read them. And who knows where the mystery POV falls into.
So aFfC gets published this fall. In October or September. Its likely that the books will be published in the US and the UK at the same time. Although the UK may get a couple of weeks headstart.
The next book will still be called aDwD. (Dany will be in it after all). This is where things get oblique again. Even though he was 100 pages away from finishing aFfC originally, he says he has 600 of an expected 1100 pages yet to write in aDwD. So clearly he feels he is putting stuff into it that he originally earmarked for the old version of aDwD.
This might refer to something he mentioned earlier in May about finding a way to deal with some characters been too young and he wants to skip ahead, while others are too active. He will just drop them.
He has offically announced that the book will take 7 books. I think aDwD will take 1 to 2 years to come out after aFfC. Getting aFfC will relieve a lot of pressure and make things easier to progress.
I think Davos will be in aDwD instead. Sam is the question.
He didn't mention changing the title of the book, so I don't believe it will change.
We all would have liked him to publish when he feels happy, but the publishers were very anxious and he has put them off long enough already. And they said the book was too big to publish as one volume anyhow (since it was still getting bigger). (I don't care if other hardbacks were published that were bigger. That is what GRRM publishers told him. And that is all that matters. Not what other publishers did). I'm happy with that. And making a tight book has a lot of advantages.