Barebulb is still going to give you a cast from off colored walls. How bad it will be is impossible to say without seeing the place.
Like segedi suggests, is it possible to fire off camera with a radio trigger?
These situations are tough and if you have to go on camera flash for key I'd use a bracket and get the light as far off axis as possibleand use some type of diffuser.
As to exposure, thats just a question of auto mode or getting REAL comfy using guide numbers on manual mode.
If you use the same focal length lens throughout it gets east to judge distance.
Why an 18mm? I know the place is probably tight but it wont exactly flatter guests and good luck if anyone ends up in the corner of the frame.