When SerializeEvents is false, do filesystem operations for the same file fire on one thread?

All filesystem operations for the same file will indeed fire on one thread (i.e., are serialized), but this thread is not always the same. The thread is picked from the worker thread pool and you should not make any assumptions regarding which thread this is. You can rely on the fact that, if an application resized a file and then wrote data to it, your code will first receive the SetFileSize event, and only after completion of said event will it receive the WriteFile event. But the WriteFile event may be fired on a different thread than was used to fire the SetFileSize event.