Regarding simulated umbrella lights - I don't think it is really necessary to simulate the 3d shape of the umbrella and bounce light around in it. You could use a simple glowbox but feed an intensity mask into the Ambient Color slot of the glowbox's surface node. For example, something that blackens the corners of the box would give you a proper imitation of the shape of the light and, if you really wanted to, a radial gradient centered on the mask would give you the effects of any non-uniform illumination of the umbrella. This would, of course, only work for the diffuse component of the illumination of the subject and then only with IDL turned on, but umbrellas are for making soft, i.e. diffuse, lighting anyway and the IDL algorithm will then calculate the falloff in a statistically correct manner. Mostly, I don't think you'll see significant differences.

Whichway