We would need a camera brand in order to recommend a specific speedlite. However, any top tier or second tier speedlite would probably be up to the task as far as its ability to illuminate. Top end units typically have faster recycling, battery pack provisions, and other bells and wistles.

As for attaching the CST to the camera with a speedlite: rubber bands and/or velcro. I use both, and each works well.

Well, a lot of will have to do with the venue. Remember how light intensity falls off the further you get from it. So if your light is aimed at the reception floor, those closer to the light will get more light falling on them than those across the room. However, a kind of high ceiling can be close to the same distance from everyone. Very low, very high, colored, dark, or absent ceilings are less than desireable, so you may have to go the direct flash approach. With the size fo the room vs the size of the modifier, the size of the modifier will be of little consequence in most cases. I would go with more manageable over more size. I would also research several approaches before taking the word from any one photographer, perhaps hybridizing multiple approaches.