I'm really sorry but I have no experience of unlocking the SGS from a network lock. I always buy my phones unbranded and unlocked because of the hassle you are going through right now.
One thing I would definitely recommend though is backing up your /efs folder. This contains encrypted data which is unique to your handset, and covers the IMEI too. Any messing about with your nv_data.bin must carry an inherent risk and I'd hate for you to brick your phone.
I can't offer you advice because I would just be guessing and unless I've done something myself I don't like to tell people how to do something.