Since you don't have the working space, straight flash on a bracket. And make sure the flash is about a foot away from the lens axis, to eliminate red-eye.
You might be able to arrange to put up a remote flash on a stand, if no one knocks it over. That will give you off-camera flash angle. And it won't take up a lot of space and it won't block the vision of the audience like an umbrella would.
Don't bother with diffusers. In a large area like that, where is the light going to bounce off of? Just a waste of light and battery power.
The only use for a diffuser would be for a wide angle lens where the 555 does not throw a wide enough light pattern to cover the lens. Then I would use a FLAT diffuser over the front of the flash, not a tupperware bowl sending light in a 360 degree circle.
What I would do is to experiment with various DIY light modifiers to see just what the effect is in a BIG room.