I am no pro especially in regards to weddings, but have shot one before. Here is my advice
I would not use flash during the ceremony if it can be avoided. I also would not go wider than f/2.8 unless its during the formal portraits. You will kick yourself afterwards when you look at the shots on a big screen and realize most of the shot is OOF. I don't think that shutter speed should be an issue during a typical ceremony. I would shoot in Av mode at 2.8 if needed, and then up the ISO to compensate accordingly ( I wouldn't go too terrible slow though, but people typically walk the aisle slowly and then everyone is pretty much standing in place the whole time. Speed becomes a bigger problem during receptions when people are dancing and moving about much more. During this time, I would use Tv mode to get a decent shutter speed along with ISO. Then let the camera choose the Aperture. (it will most likely choose 2.8, just make sure if you do this, that you don't use the 50mm 1.4 or it will probably default to wide open.
If you don't know already, get comfortable with the exposure comp. That way everytime you enter a new area or room with this method, you can snap off a quick test shot, glance at the histogram and maybe over or under expose slightly for that room. ( if there are candles and other wierd lighting, it may throw off the automatic metering depending on your mode, so you don't want to just ASSUME the camera is getting it right.
As for the flash, since I assume it will be on camera, your idea seems decent if it will in fact freeze people dancing. Afterall, letting in ambient light will help avoid that "Flash" look.
Thats just my 2 cents. Take it for what its worth.