Presumably, the light on the stage isn't changing during the ceremony. And, presumably, you'll be able to stay in the same spot during the whole ceremony. Therefore, manual exposure and manual flash power.
If the ambient lighting isn't changing, and the distance to subject isn't changing, you don't need any of the automatic modes - they're just going to work against your intention to have the same exposure across the whole event. The automatic modes are great when ambient lighting is changing, and ETTL is a boon when the distance from flash to subject is changing. But those aren't the case here, so you don't need the automatic modes.
- Use 1/125 or 1/160 to freeze the motion of slowly moving people. They're not posing, so anything under 1/125 will risk motion blur. But they're not leaping and dancing about, either, so 1/400 is overkill. So, 1/125 to 1/160 will be the sweet spot.
- Use a relatively wide aperture to try to separate the subjects from the background of the stage. f/2.8 for best separation between subject and background; maybe a stop smaller for more reliable focusing and maybe a little more image quality (I don't know how the 17-40 does with sharpness and CA when it's wide open).
- Set the ISO so that your non-flashed exposure is underexposed by about a stop.
- Set the flash power to add just enough light to properly expose the subjects on the stage.