Only way to answer that question with certainty is to re-read every word; otherwise, in a series this large there's a good chance you'll miss something important. In a book as large as AMoL will be there's a better than even chance he'll make a significant continuity error, but it would be surprising if he DIDN'T do a re-read before the final book since he hasn't read it in three years. A lot of people do re-reads before reading a new WoT book, because there are a lot of important plotlines with their own sets of important details to track, so it's only reasonable to expect he wants another re-read before WRITING the next book. Sanderson has stated that he takes his obligations to Jordan, Harriet and the fans (no doubt Tor as well) very seriously, and it's to his credit that he wants to get the ending right, or as right as possible. I don't fault him for that, and he's probably proceeding very slowly because he's mentally and physically making notes on how to integrate what he plans to write with what's already written. As long as it's published by 22 December of next year we're OK, right?