It's even more amusing. It's possible to distribute the writing function between an arbitrary number of entities and dump the editing on the guy who has to do the interaction function.
This way it is possible for a story to have any number of entities who can be called "Gamemaster", in a fashion.
What I think does happen in this particular case is that Bernkastel
wrote her story, liberally swiping pieces of kakera all over, but left the conducting part to someone else - not like Will ever did anything unexpected anyway. As far as I can see, he was railroaded for the entire duration up to Claire story and I expect this holds afterwards as well.