So Sanji was able to know the intention that Nusjuro will bite him, yet not the intent of how or where the attack will come from? That doesn't really make much sense tbh. He should know where Nusjuro's attack will come from as Nusjuro intends the attack to come out of his torso. And even still Sanji didn’t put up any defense against the attack.
Stussy simply tapping Sanji's face made a 'clink' sound effect, indicating that she's touching something metallic, yet his eyebrow's normal. His exoskeleton is likely always active. The swirl change likely indicates a change in Sanji's mentality. Anywho, Sanji permanently has enhanced durability regardless.