Jim Port - Actually, yes - at the switchbox, the grounds and whites are pigtailed, while the blacks are on the switch. I suppose that IS the easiest way to do it - just wire the DW separately.
AllanJ - if I do not have to fasten to studs, do I at least have to secure them to the box itself?
Another question - The house was built in `72; could the receptacle box used then possibly not have prepunched knockout holes? I wasn't able to see any, but it could just be that it was pretty.. rusty.. in there. That's what made me think of replacing the 12/2 with 12/3 - i won't need new holes
It was pretty solid-looking metal to me, so without those prepunched KOs, I'd need some good muscle to start another one for new wires.