Almost no one uses LuxRender or Cycles commercially to the best of my knowledge (or appleseed for that matter). Hobbyists use them, which is cool.
I'm in Los Angeles and work in the industry. Here, people use Renderman, Arnold, and in-house stuff mostly. I don't think Renderman and Arnold are used much outside of VFX. Arnold in particular is pricey, but works amazingly well.
Disney has Hyperion and Weta has Manuka, although I'm not sure if they license them out to other shops.
Not that I know of. Many houses also have specialized in-house renders for voxels and occasionally, fluids.
Hella cool! Who is rolling their own outside of Weta?
DD? Animal Logic? Double Negative? Blue Sky is super proprietary, I'm curious what they are up to...
But more studios have their own full renderers these days:
Weta has Manuka
Disney has Hyperion
Animal Logic has Glimpse
Framestore has Van Damme
Sony has their fork of Arnold