ADWD is, in effect, structurally complete. That means about 90% of the book is finished, edited, done and dusted. They've even completed the maps and appendices, and typeset the completed chapters.

The remaining 5 chapters exist in drafts, partials and fragments. These need to be rounded off. Some are close, some need a bit more work.

At this time, A DANCE WITH DRAGONS is just short of 1,500 manuscript pages. That's not including the partial chapters. GRRM has apparently told Bantam USA that he wants ADWD to consist of the completed materials plus the five additional chapters. This may carry the book past the 1,530 MS page count of A STORM OF SWORDS, which Bantam have informally been using as the upper page count for ADWD. If ADWD passes that page count, that could cause problems from a publishing standpoint, so either more chapters need to go into Book 6 (which no-one wants, since that just makes Book 6 bigger and may store up problems for further down the line on that book) or Bantam and their parent company, Random House, need to agree to meet the expanded costs of publishing a novel larger than ASoS. That discussion is apparently ongoing. My guess is that, for a guaranteed million-seller, they will allow ADWD to be somewhat larger than ASoS, but not too much bigger.

Is completing five chapters by Christmas possible, given GRRM won't be back in Santa Fe for another month? Certainly. The five chapters don't need to be written from scratch. Some need a fair bit of work, others just need some mild revision to finish them off.

So it's possible. It depends purely on how fast GRRM gets back into the writing swing after returning from Ireland and Malta, and if any unexpected surprises arise during the finishing of those last five chapters.