If I retake the GMAT after sending in my applications, can I send in the higher score if a lower score was initially sent?
Will they review the application using the higher score?
They always use the higher score.
should I wait to send in the applications until after I retake the exam?
You don't need to do that. Some applications ask you if you are going to retake it. If that option is not there, just mention it seperately that you are retaking the gmat.
Will some schools want to wait for the official score or will they generally let you fax over the unofficial score?
In most cases the official score is required only after the admission is secured.