That sort of thing could well happen. Martin is a "discoverer" who outlines but also leaves himself plenty of leeway to follow the characters where they want to go as he writes. The fate of characters in finales is most often determined by what feels right dramatically and often gets changed at the last minute (take RJ for instance. His notes outlined a few deaths, and had an end point for characters who survived, but he also left a lot of "I might do either" and it was left to Sanderson to decide). It's highly possible the show writers have different needs than Martin does for specific episodes toward the end, especially when you consider the fact the show's final conflict will be seriously limited in "epic scope" and will have to find other ways to provide great drama worthy of an 8-season build up.

Martin said HBO has an outline "without all the bells and whistles", which I took as a kind of reassurance to his readers that if HBO has to go their own way, their ending will follow the general outline for his final conflict but will not ruin all the book's twists and turns as in part they're created as he writes.

Of course I think everyone (incl. Bantam and HBO) hopes Martin finishes up in time and there's no need for a different last season to the TV series.

It would be a mess no matter what. Either it will be too close to the books and spoil them for the readers, or there will be many changes and it will be very distracting when you read (as it's far more introspective and active than watching a show... you have much more time to think), or people will get pissed off by the different outcomes in some arcs.

More like almost three years, or more if he releases WOW in 2015 and does take too long a break before starting work on ADOS, but yeah it's definitely all very speculative.

The speed at which Martin writes varies a lot - four to six years per novel as happened with the problematic AFFC/ADWD is hardly his average writing speed even for ASOIAF.

I'm sure they're far more concerned at the moment with finding the most interesting ways not to get bogged down with the expansion the series saw with AFFC/ADWD than bothering about what they might or not have to do down the line if TWOW runs late, let alone if ADOS does.