1. Shoot raw
2.watch for expression
3. Yes ettl can under expose a bride shot by herself, but a bride and groom right next to each other will be fine. For just shooting bride dial just the flash up one stop, use spot meter and preflash off dress with back button, you will need to preflash every time you check picture. I have autoplay set to off and only chimp when I need too.
4. Do not use auto ISO, practice before hand with settings and ISO
5. A tripod is a must for formal pictures in church after ceremony.
6. Have an assistant to help you