Only if you use batch rendering. >>530495
Bullshit, Arnold is perfectly usable. >>530510
Mental Ray is gone, Arnold is standard render. If you want to render on more than one node you have to buy additional licenses.>>530511
Depends. Its way easier to set up, and it gives a perfect result given enough time/ renderpower.
But its not perfect for architecture, especially indoors.
It runs only on CPU so its slow but very stable even with billions of polys.