If we still have a list of alternates, why not give them a shot to make the book? Put in a cutoff date, and allow the alternates to submit some images and descriptions for consideration. If the primary participants don't submit by that date, take any of the alternates that are ready to go and drop the primary person.
If we are short of alternates, have those that already submitted their two photos pick out an extra image or two and submit those. First come first served, and use them to fill the extra pages. I'm sure those that already submitted could scratch up another image if need be. Does it have to be 25 photographers? Why not 21 filling the same number of pages if we have to in order to keep the project on track?
We shouldn't let those that can't submit by the dealines slow the project down for everyone else. It would be nice to have this available for christmas presents for those so inclined.