It's not a "terrible example". What does that even mean? It's made pretty explicit that the proximity of Nihilus' servants to him (like those on the Ravager's bridge for example) had that kind of effect. All living beings in the Empire are touched by the force, and just in case it slipped your mind, there are a lot of Sith that "non-Force sensitives" are constantly around.
Why would they need a new face model? This doesn't make any sense, so I'm just gonna ignore it.
I meant that does it explicitly state anywhere that it was, in fact, dark side corruption and just some other kind due to Nihilus' power? Visas wasn't affected by it at all, after all, at least she had no visible signs of it.