I'm not too sure, but I suspect that most of the authors who work for Del Rey don't actually sit at their keyboards and type out all their novels themselves. I presume that they dictate to someone, or a few someones. I assume this anyways.
So Allston could be all, "And in Backlash I totally want Daala assassinated, and I want Myri Antilles to get married and Luke and Ben will visit the Jensaarai and a Jensaarai adept will totally hit on Ben and Luke will feel awkward and it will be funny."
And give the germs (and even full-blown ideas) to some sort of scribe or something.... I don't know. I presume that Del Rey are going to do whatever humanly possible to keep Allston on this series and to shuffle the schedule if need be to accommodate him. I highly doubt they're going to kick him to the curb and replace him with someone else, unless things get really bad for him (which seems unlikely at this point). Whatever cynicism or negativity you want to throw at Del Rey, it's clear to me anyway that they seem like loyal employers in terms of their author roster and the execs at Del Rey aren't the sort of employers that will turf their authors when they have health issues.
I also recall that Mr Stover had some sort of health issue around the time of LSATSOM, which was why the schedule was moved around regarding that novel. And look at LSATSOM; from reading that you would never guess that Stover was having health issues while writing/publishing it because IT WAS SO GODDAMN AWESOME. I assume the same will apply to Mr Allston's FOTJ entries. That's my two cents worth. Take it or leave it.

















