I like how whenever they show Ryuko's mother and father in the past, they look like they're drawn in a completely different art style. Like something out of a 70s-80s anime.
I suppose it's right up there with claiming to have said something you in no way said.
I thought that was a piece of Sanketsu that she turned inwards.
Hey...that's a good question.
I can only assume Ragyo's explanation about herself also applies to Nui. That as long as there is a single thread still attaching them, that they can immediately regenerate.
Maybe Ryuko's dad actually cut her eye OUT of her face, and cut any connection the eye had to her body.