Machines can sport physical changes without mutation, it's called modifying.
The snipers have a pocket dimmension too. This is not the eye of terror.
Like all other races the necrons abhor the warp. They constantly keep a look out for it. I think they would have noticed if any of them were warp-tainted.
So I must disagree with you on this, as it makes absolutely no sense whatsoever based on what I have read. They have a virus, which makes them act in a different manner than Khorne followers. They do not seek their own destruction, just life.