First off, I doubt the expanded playtest will last 6 months running from level 1 to 30. More likely it will be divided up over multiple groups. Some will be asked to create 1st level characters and play them through 4-5 levels. Others will be asked to create higher level characters, or maybe given pregens,and bring them up another 4-5 levels covering the entire range of levels.
Second, if they have any brains, they are already doing compiling and editing especially of sections that are less rules-intense, or simpler and less likely to need significant changes following expanded playtesting.
Last, I think you would be surprised at how quickly the books can be printed once they are ready. Harry Potter books I believe were being printed a few months in advance of release, but they were dealing with having to have millions of copies available on the release date.