To copy data the drive doesn't need to be formatted for windows, maybe some data was corrupted since that bootcamp partition was faulty, sorry about that.
On your windows installation usb stick, is there an option for you to upgrade from windows 7 to windows 8? That could work.
Some people have reported issues with bootcamp partition on fusion drive, did you take the apple care plan? If you did, I would call them maybe they might have more answer for you, or even better take an appointment with one of their so called genius if you have an Applestore in your area.
I ended up just getting Windows 7 and redownloading everything I could remember from memory. Not what I wanted to do, but I don't have Apple Care so it's ok.