Any way to backup FS and main before flashing k750i

Use Far manager.You can backup MAIN,FS,GDFS and EROM if you know the adress.

Oh btw.I've read that in GDFS is written if the phone is locked,...

i dont know but u can write down the firmwares on your handset before you flash and then backup GDFS with XS++ v3.1

if anything goes wrong during flashing just restore GDFS and flash original firmwares

Yes You Can But If Anything Goes Bad, It Better To Restore Original Firmwares And CustomPacks And In The End Restore Your GDFS.