The programming language doesn't matter, and the virus is only as good as the coding. Is one PL better for writing virii? Maybe, maybe not. It's a personal choice I would say.

No, they aren't necessarily using unix or any particular OS.

It needs to be said that hacking, virus writing, or any other computer penetration techniques are not cut and dry. Your questions tend to have the form, "if this, then this happens or has to happen" and this is not necessarily true. Keep an open mind and learn from what you see. The moment you think someone hacked into your system using Unix, you'll be disappointed to find out they used a window 3.1 pc in the back of a library somewhere in la la land.