At the end of the day, A Dance of Dragons is merely the
title of the 5th book. If it will now contain stuff from AFfC then whatever can't fit into book 5 will go into book 6.
I see where you're going with this premise but the books aren't written yet so he can do what he likes. He is the author after all....
It will probably mean that we will definately not see Dany in Westeros before book 6 though...